When John and Marcie were good friends in middle school nobody seemed to care. When their relationship developed further in high school, their parents suddenly objected. Especially Marcie's father, but well, he's a violent idiot. John's mother Karen had objections to, not quite well defined objections at first. She's a manipulative bitch who has turned John's dad Peter into complete wimp by now. Eventually when pressured, they claim that John and Marcie are half-siblings, specifically Peter is Marcie's biological father too.
Thing is, Marcie's mother is dead and girl confess she believes her (step-?) father killed her mother specifically for her not being his daughter (of course she can't prove it). She kinda knew that long ago, but doesn't know is it true or who her father is if so.
However, Peter's life should have been interesting at some point, because John's aunt, Karen's younger sister Olivia also claims that her daughter, John's cousin Inez who's about the same age (and John finds her especially attractive too) is also Peter's daughter and John's half-sister. John does believe that and can accept.
The question is about John and Marcie. Are they in legal trouble if hook up like John's mother is trying to insist? I suppose they could do genetic testing in hopes to prove that Peter isn't Marcie's father, but can they even do that without Peter's collaboration? Then without that all there is are claims by people who wasn't actually involved in making Marcie.