Sorry about your situation. My older sister went through something like that. She married a guy with two very young girls. Their mother was a real slut and didnt want them cramping her style. My sister took them in for many years, but once they got into their mid teens, they went back to their real mother. I think mainly because their real mother didnt give a **** and let them do whatever they wanted, go wherever they wanted, etc. She bought them booze and smokes, etc.
Now that the girls are older, they've sort of gravitated to the middle. My sister thinks they appreciate her now, but I think it's more like the girls are just playing both sides, because both sides want to see the grand kids, and both sides help out when needed, with money, food, clothes, etc.
Guess its true what they say about blood being thicker than water.
Wow, we really have a lot of similar experiences - lol.