On your brothers profile, go into edit, on the right where it shows the spouse, you need to select the add/remove spouse and at the bottom, you can remove her.
This frequently happens when you add the 2nd parent to a child and use the GO button at the top.. Unfortunately at the bottom of that page, there is a default statement that says the parents were married.
Just remove the spouse, but you probably need an explanation added to all 3 profiles that her parents were not / never married when child was born, so no one connects them again.
Add information about the adoptive parents in the biography.