Unfortunately I've had this too with a step parent. I turned off romantic calls and invites in my Game Options and felt thoroughly creeped out. The household was created with a Mother, her husband and her teen and YA daughters from a previous relationship. The teen lived with her mother and Stepdad, I used the relationship label to label him Stepfather and used the Adopt as Care Dependent social interaction. She then aged up to YA and left for University which is when he randomly invited her out. I have Lovestruck and I don't know if that affects it. She didn't have a high friendship bar with him and her storyline was that she didn't get on so well with him but they weren't enemies, just acquaintances.
I've had a look through the bug reports because I thought it might be one but couldn't find a report. So maybe feedback for now.