Here's the story: I have a stepdaughter who will be 11 soon and a DD who will be 4 soon. We are currently getting SD every other weekend and will get her about 5 weeks out of the summer. SD is planning on going to the beach with me, DDs and my family this summer. Her brother went with us last year and they know we go every year. Here's the problem: SD and DD1 do not get along very well at all. Sometimes, they play together so nicely but most of the time, SD does what I call micro-managing of DD. She does more correcting of DD than I do. I am a very laid back parent and SD likes to cause drama, instigate stuff, see how far she can push things. She will often set ultimatums for DD (if you don't clean up your toys, we won't go XYZ) and will even pick her up and put her in time out if DD is not doing something that SD tells her to do. It has gotten so bad that if SD has a 4 day weekend, I usually send her back to BM after 3 days because I am tired of the constant fighting. I really don't want to spend a week at the beach refereeing. Is there any way to fix this? I have tried talking to SD, telling her that I am the mom and giving her the option of moving to her room or going outside if DD is bothering her. I have tried talking to DD and telling her that she needs to make sure she is not bothering SD and she listens to SDs words when SD wants time by herself. I generally don't have any other problems with SD.

Re: Parenting advice needed
First, this is not a half-sibling issue, but an older sibling issue. I did it with my younger sister and we both did it to our younger brother. Same thing does with every single Older Sibling I have ever run across.
So sending her back to her mother's is not the answer...parenting her is.
Which brings me to point #2 - where is the mutal parent in all of this? What does your DH do when she plays mommy?
Which brings me to point #3. Once you and DH are on the same page, then you can most definitely make any rules about ANY behaviors that you deem inappropriate (though this seems a bit incosequential).
Have your DH tell SD that she is no longer allowed to be bossy and that if she does get bossy with her sister, she will be sent to her room for 10 minutes for the first infraction of the day, 15 for the next, and so on.
Though the reality is, what she is doing is really nothing. SO WHAT if she says "Do XX to get YY"...unless YOU the parents, actually follow through with HER threat, what does it matter?
"I usually send her back to BM after 3 days because I am tired of the constant fighting."
Not for nothing, but parenting is ALWAYS about repetitive arguments. And there is MORE of it when you have siblings. If you cannot handle this now...please dont have another kid. Becuase your not going to be able to send your own kids away after three days...nor do you seem ready to send your 4 year old away right now...
Honestly, I think you need to learn to address it. If SD is mistreating DD then you should step in. Having more than one kid means you're going to be a referee. I completely understand the frustration with the situation but I don't think it will get better until you step in and take some action. It seems pretty normal for me that an 11 year old girl would want to take on a parental / authority role in a 4 year olds life. With her being 7 years older I think she should have some authority over your DD but not to the point of ultimatums or time outs. More along the lines of "no, you can't hit people"
If you go on the vacation without SD that would be very hurtful. If I were in your shoes I set clear limits with SD. Explain that there are some things she's welcome to do with DD but other things like time outs and ultimatums are for adults only. You could even go so far as to identify consequences of her overstepping her bounds. Then stick to them.
My DD does this with her younger brother (2 year age difference).
I call her Miss Bossy and remind her that *I* am the boss, not her. And yes, I am shouting b/c nobody makes the mistake of thinking they are the big dog in my house.
When she sends your dd to her room, send HER to her room - and base it on age (your dd is 4, would get 4 minutes, SD is 11 so she gets 11 minutes) and tell her she is in there until she realzes that you are in charge of discipline in the house. Seriously - - you have a rule that is "don't interfere with my parenting," and she is breaking it. If she picks her up, tell her "DD doesn't like to be picked up, and I don't like it either."
I also question why they are playing together. An 11 year old will not naturally want to play with a 4 year old. Even though they are siblings, it's ok if SD doesn't want to "play" with your DD. That would happen even if they were full siblings. Give SD the freedom to do her own thing. Also, use the "free babysitting" that your parents provide to spend some one-on-one time with SD (or do something when DD is napping).
As long as SD is not babysitting, I am not sure why this is a problem.
You are the issue here. Not the children.
But she knows she went home early. And it's not that difficult for her to put two and two together that you seem frustrated/stressed when she's around and all of a sudden she's going home. I mean seriously.
seriously, why do you ALLOW this? an ELEVEN year old does not have the authority to take away someone else's toys or to CANCEL TRIPS (seriously, that's ridiculous) what makes her believe she DOES have this authority?
Yeah, you have to......be a parent.........shocking, I know.
I completely agree with the PP that said you need to step up and parent the 11 year old. Our older kids are not allowed to parent the younger ones. The things you are saying she is allowed to do would never be allowed at our house. If one of the kids so much as tries to correct one of their younger siblings, we intervene and remind them that they are a child and they need to leave parenting to the adults. SD in particular used to try to parent the boys. Consistently reminding her that it is not her responsibility and that we just want her to be a kid has drastically reduced that.
Sending your SD home because of normal sibling behavior is not good. All older siblings will try to boss a younger one around. Your job as a parent is to teach them what is and is not appropriate.
My 5yo SD does this constantly with our 2yo DS. I did it with my little brother, and so does every other older sibling who even has a slight interest in their younger sib. Is it acceptable? No. But in a psychological light, it means she has a care and interest in her younger sibling. I'm not saying it should be allowed by any means.
Why on earth would you send her back early, ever, unless in an emergency? And why are you the one making that decision?
I rarely take advice from my MIL, but on this particular issue, I have. And it is doing wonders for my sanity, as well as my kids' relationship with one another. She told me to let them work it out and only interfere when necessary (older one starts implementing punishments or there is danger of injury). I find that the more I step back and only referee when necessary, the better they both are the next day and the next, each learning the social behavior required in this kind of situation. That being said, the accepted social behavior cannot be learned if not guided. I cannot count how many times a day I ask SD "Who is the mom here?" or say "You are a child, not a parent." But I typically leave it at that. For my kids, a reminder is sufficient. But that is because we have parented every day rather than just sending SD on somewhere else.
If it really driving you crazy, maybe you are micro-managing the situation as much as your SD is. I say that to be constructive. I have a problem with micro-managing, as well, and my SD gets worse about harping on her little brother when I am already in control mode, which only adds to my frustration. Those evenings are not very peaceful, and even if I get mad at him at the time, I love it when DH takes over for a little while and tells me to go chill out because I'm being witchy and driving everyone nuts. The kids are so much calmer when I am calm, as well.
My SD(16) does a lot of the same things. Theropy helped a ton! SD was able to get her feeling out and the doctor was able to correctly point out the negative/bossy/controlling part and redirect her without her getting defensive. So as much as I agree with everyone that it is normal and your household needs clear punishments for this SD may also be a bit extra and need the theropy short term for the controlling/mean actions.