I am familiar with that one. I am also in a second marriage so my three kids are more objective (all adults and out of the house). His son is very supportive but after I shared my concerns with his daughter, within a month she asked him to cosign a car-loan and asked for her and her daughter to move in with us but to not tell me! Lucky our policy is to always discuss our kids but each (biological) parent will do the talking to their kids. You are right and it can be sweet and bitter! Hang in there.
Wishing you peace and strength