Any step-mom/first time moms out there??

Hey group, needing a little support.  I'm a step-mom to two wonderful kiddos DSD 11 and DSS 8.  BM has been nasty since we got engaged (basically parental alienation to the kids, criticizing everything, talking bad to them, etc).  We have them 50%. 

I struggled with infertility for many years with my ex-husband.  When I married my current husband he knew what I'd gone through and that I wanted to have a kid(s).  So, IVF #2 with him worked and I'm pregnant with a girl!  I couldn't be happier!  The problem is, I feel like my joy is crushed on a daily basis.  The kids were SO excited to find out about the baby. Then BM stepped in and started blaming the baby for things.  They constantly come home telling us what she said.  And now she's gone from throwing DH under the bus to throwing me under it too.

And on top of it, my dad is telling me I hurt my brother's feelings because I posted a 20 week u/s on Facebook and I didn't text him personally.  He knew I was pregnant; we are NOT close, and we don't even live in the same state.  

I just feel like everyone is crushing my happiest time.  I never thought this was going to happen and now that it is, I want to be able to be excited and happy and not feel sad like I do now.  

Can anyone relate? Does anyone have advice for me?  I talked to a counselor, went back and talked to DH, but he feels trapped in getting BM to stop alienating and damaging the kids.  I'm concerned for how that's going to impact my future with DH and DD.  Help!!