How do I maintain a relationship with my child’s siblings when the other parent is abusive?

We are in the final stages of adopting our daughter. She has been with us since she was 4 weeks and she is currently 2. Throughout the past 2 years we’ve made every attempt to be available to her siblings who are with another family. We’ve become known as Aunt and Uncle. We love the older kids, but as time has gone on it’s become harder and harder to maintain a relationship with the other “adoptive” mother. I say “adoptive” in this case because the children are placed with one of the birth fathers and she is the paramour of this father. They are not legally married and they only have guardianship of the kids. As time has passed I’ve come to realize I really dislike this woman. We have nothing in common and have very different views on child rearing. In my opinion she is borderline abusive to the children. The birthmother died a couple of months ago, but I know prior to her death her main goal in life was regaining custody of her 4 older children. How do we maintain a relationship with the children with this woman in their lives? It’s to the point it’s causing arguments between my husband and me because he feels strongly about making sure our daughter knows her siblings and I do too, but my patience and sanity can only take so much from this woman. She is rude and demanding. I work in the Social Service field so when I say borderline abusive I know what I’m talking about. There is nothing there that would make Children’s Services remove the children, but there is little there that I see that is nurturing and loving. I know that if I approach her with my concerns she will be beyond angry and I don’t want her to take it out on the kids. I also know that she will not allow us to see the kids if I make waves.

She has a very tight family that can come across as agressive. The father in the situation is very passive and does not have a lot of voice in the situation. Does anyone have a suggestion? I’m at the end of my rope and am really ready to be done w/ the disfunction, but I want my daughter to know her sibs.