My husband has a son from a previous relationship and up until we got together (I had a daughter as well fom a previous relationship) and had 2 more daughters together he was an only child and her only grandchild. She would pick my step son up on Fridays from his mother's house (that was fine because then I didn't have to put my girls in the car and drive a half hour to and from) and then take him out to eat and drop him off with a bunch of things she had bought him. In the meantime my daughters didn't understand why she never took them anywhere or bought them things. I finally had to have a conversation with her about it and told her that if she wanted to continue to do that then she needed to do for all the children, not just him or else keep what she buys him at her house. She was over sensitive about it and it turned into a huge fight. It stopped that though and we never had that issue again! She used to interfere with how my husband was with his son and tried to undermine his authority as a parent but I finally got him to stand up to her and she doesn't do it as much anymore or at least not intentionally, when she begins we quickly remind her who the parents are in the home and that he needs to follow our rules and respect us and that if she doesn't give my husband respect as a parent in front of him that he will never respect his father. Things have been a lot better though.