Yeah they were probably just friends, but you know maybe the MC will be hit by another piece of drama in addition to that.
People tend to find love interests via their social networks, more so during the time that the MCs mom was young and hung out with Jill’s mom.
It’s entirely possible that Jill’s dad for instance at one point had a fling with the MCs mom, without it resulting in any children, before ending up with Jill’s mom. That could be a sticky situation, or maybe Jill’s mom also had a thing for the MCs dad and as a result has a massive grudge against the MC or wants to bang him.
Plenty of opportunity for drama without any incest.