I am a mother of two boys. My oldest being 24. I am also a step mother to two daughters.
When ever there is a break up with someone... the ex's still try to stay in contact with us parents. They ex's do that to keep up on our kids. I just find it hard to tell there ex's to move on and that we (meaning myself and my husband) can no longer be friends with them. I just try to slow down the contact between us.
My oldest son and my youngest step daughter really bring there partners around and involve there partner in our families life. So, it is hard to just cut them off.
My oldest son has been upset with me over this. However when he is with a girl he wants me to include them and do things with them. I have asked him not to bring them around unless they are the right one. But he doesn't know if that girl is the right one or not. The youngest step daughter well she is a free spirit and nothing seems to bother her.

so my point is from a mother / step mother... It is hard to cut people out of my life that want to be in it so badly.

I don't know if this is what is going on with your step mother or not. Just trying to give you another perspective as to what maybe going on.