I am a birthmother whose family did to me what you are trying to do to your sister---tried to force me to choose to place the child with family once they knew I wasn't going to back down on the adoption.
I was yelled at, made to feel guilty, given the silent treatment, got hate letters, and basically got told what a scum of a piece of human being I was for giving my child to complete strangers. In trying to show me how VERY much my family loved my child, they showed me that they didn't care very much for ME, because they were putting their own feelings above my own.
It took me YEARS (and a lot of counseling) to forgive them.
And the fact of the matter is that while the father of the child can contest the adoption, no judge is going to let him contest it only so that he can give the child to his/her mother's family to raise, in direct contrast to the mother's wishes.
ESPECIALLY since you will be ripping that child away from his/her parents, who love that child very much.
What you want is very selfish, and essentially shows that you care nothing for your sister.