My sim can't do anything romantic with her boyfriend

Okay so I had this family, a mother and two daughters, and both daughters got into relationships, one with a girl vampire and the other with a boy vampire. So then the mother started dating the father of the boy vampire, who was also the step dad to the girl vampire, and the mom of the daughters married the vampire dad and they all moved in together and now the girl vampire and daughter can still kiss and stuff, but the boy vampire and the other dad can't do anything romantic even though it still states them as boy friend and girl friend, is it because they are considered step brother and sister? but like why does it still call them boyfriend and girlfriend then? Is there a way to fix this?