I know I vented about my sister once, several months or maybe even a year ago. Shortest background possible: She's 5 years older than me, and has a blended family as well. She and BD were never married and she found out she was pregnant after BD moved about 8 hours away. She started dating someone (who is now my BIL) before she found out she was pregnant. She flew out to see BD and told him she was pregnant and he moved back shortly after my niece was born so that he could be a part of my niece's life. Things were really rough at first and there is still quite a bit of hostility among the 3 of them (my sister, BIL and BD). To be completely fair the resentment is on both sides, largely because of the way BD behaved when my niece was born and him feeling threatened by my now-BIL being so involved. But really, when my sister went in labor it was my now-BIL who took her to the hospital and they have lived together since shortly after my niece was born. He is every bit a father to my niece as BD is, except for biology. My sister is no saint in this and she has put up her fair share of roadblocks to try and keep BD away. My niece is now 10, my sister has another child with my BIL, and BD is married with another baby and one on the way. My sister pitches a fit about everything BD does, and she's a complete hypocrite. Her shenanigans rival BM's and it's infuriating trying to deal with her. Truthfully, her mindset on all of this is so similar to BM's, it's scary. Yet when I tell my sister what BM does, my sister says how awful it is of BM to pull that crap. But when it's my sister doing it, it's totally justified.
My current vent: she called me the other day trying to figure out Thanksgiving stuff and that led to her asking when my niece will be allowed to say she doesn't want to visit BD anymore. I told her that children really don't get a say. Their opinions are taken into consideration, but seeing as how BD currently only has 1st, 3rd and 5th weekends, it's unlikely a Judge would lower the visitation any further. And BD is consistent on taking his visits and frequently asks for more time. My niece has her own bedroom at BD's house, she's not being neglected.... there's genuinely no reason to change things. My sister starts telling me things that my niece has told her about BD's house and that my niece never wants to go visit him. Given what just happened with K and us finding out she's telling stories and stretching the truth with BM, I told my sister to take what my niece says with a grain of salt and told her what's been going on with K. Of course that's not what's going on with my niece. Of course these horrible things are going on at BD's because my niece would never lie to my sister. Of course my niece would never tell my sister what she clearly wants to hear or manipulate things. Of course BD and SM are these mean vicious people that my niece claims they are. I don't buy it, and I told my sister I don't buy it. I have spent a lot of time with BD and his new wife. They have extended invites to my kids (K included) to parties at their home, they have allowed me to pick up my niece from them on their time for K's birthday party, they have let DD spend the night with my niece and BD used to come to DS' elementary school (when we lived closer) and let DS help him with PE class. These are not "bad" people. But my sister is just so self-entitled and honestly awful about this blended family stuff that she doesn't see any of that.
This stuff has really made it so that I can't stand talking to my sister. I've told DH some of the stuff that my sister has done and he loathes her. I pray that I am never called to testify in my sister and BD's case because she won't like what I have to say. I know there is no reasoning with her, just like there is no reasoning with BM. It just sucks.

Created by MyFitnessPal - Free Weight Loss Tools
Re: Vent about my sister
I would have a hard time listening to that especially knowing that your niece's BD sounds like a good person and takes good care of her. I would tell her that for you to be able to have a relationship you can't listen to her rationalize how horrible BD's involvement in your niece's life and are losing repect for her. I'm sure she tells people who don't know BD the same things and they feed into it and tell her exactly what she wants to hear.
I'm sorry. It is hard to see a siblings weakness and see it effect a kid you love.
I'm really sorry you are dealing with this.
I know what that's like but I was hearing both sides of the story with my two nieces. From my X best friend who is now married to my brother and I am still friends with my nieces' mom. They don't have a C/O and X was very reasonable with letting my brother and XBF get the girls. X was going by the guidelines in our state until the girls started telling her about things that XBF was saying to them and how they were being treated by her (X saw signs of them not wanting to go but didn't want to question them, they then opened up to her and told her what was going on). X still allowed my brother to visit with his girls at a relative's house but XBF wasn't included after the first few visits so it was automatically "she wants him back". XBF even stopped giving my brother a ride to see his girls. It was just really pathetic. I'm not in the middle of it anymore but I know their mom is doing what's right for them and it was my XBF, now SIL who was doing wrong. She even sold all of their toys and clothes when X made it clear that the visits would be temporarily supervised to see what was going on with the girls.
I have left a lot out but I can relate and your last paragraph... sums it up for me with my SIL. If they were ever to go to court to get a C/O put in place and needed witnesses, I'm pretty sure I would be one and there would be some apologies and ass kissing coming my way but I still wouldn't lie. Those girls are my family too and they, just as any child, don't deserve to be mistreated especially by some jello, paranoid B. I think XBF already had it in her mind that X wanted my brother back.. and she was taking it out on my nieces. X is an extremely nice person and she is very happily married.
It's a really tough spot to be in.. I can't imagine still going through that and dealing with BF issues of my own. You have all of my sympathies.
My Loves= SD 18 SS 16 SS13 DD13 DS10 SD6 SD5
Can you tell her it makes you uncomfortable when she brings that stuff up and maybe she needs to be in therapy? Or maybe you could be horrified and say "oh sis why would you say that?! Not visiting her bio dad would be a seriously sad depressing day for any little girl to deal with, let's hope that never happens."
Maybe change what you say to your sister. Don't use her as a place to complain about BM. When you're with her, say "it's such a shame we can't work together for K's sake."
Ditto venting about DC - tell her how lucky she is to have an involved, loving, responsible (and local!) parent for her DD. Skip the complaints about DC because I'm sure she has plenty about her daughter's BD. For al you know, she thinks you are "lucky" that DC is uninvolved and that your DH can step into the role of father-figure without any pushback from DC.
I agree with what Illumine said earlier about people starting to believe their own lies. IMO that's what is happening with your sister.
Created by MyFitnessPal - Free Weight Loss Tools
My DH's BD walked away from him as a kid and his SF adopted him at the age of 6. It was a situation where BM (DH's mom) did everything possible to push out BD until he finally just gave up, thinking he was doing the right thing to end the drama. BM pretended like BD never existed and went on with her happy life.
When he turned 18 he went out and found BD and contacted him and they have been working on a relationship ever since. DH had so many issues regarding trust and resentment for all of his parents. He blames his BD for walking away, his SF for never treating him like his child and his BM for putting her own needs first. His BM still pretends like BD doesn't exist and it puts so much more pressure and guilt on DH for trying to have a relationship with BD. It drives me crazy because even as adults his BM can't see what it does to her son to create so much tension over choices she made. Luckily my DH is a very strong individual and did what was best for him even if it meant his BM didn't speak to him for a long time because of it.
Bottom line is that kids (even adult kids) have a need to be accepted and loved by their bio parents no matter how crappy they maybe. Anyone who thinks it will be much easier for them to have the other parent disappear is delusional. It might be easier for them temporarily but is so much harder for the child to deal with the rejection and feelings of unworthiness.
Your sister is a shit-stirrer and she likes drama. Probably thrives off of it.
There isn't much you can do about it but tell her, "I love you, but I really can't discuss your issues with BD anymore because I completely disagree with how you are consistently handling it."