You shouldn't sleep with your family members because that's incest.
Frank met Sara through mutual friends and has romantic feelings towards her, but Frank recently heard they may be distant cousins.
Frank wants to find out if he is truly related to Sara.
Frank does a search heritage search online to see if he is related to Sara and finds a connection they are family members.
Franks asks his mother about Sara and finds out they are cousins, but doesn't tell Sara and ends up having sex with her.
Sara asks Frank out, but he tells her about the information, and they agree to act towards each other in a respectful manner.
Sara ends up meeting Frank's mother and finds out they are related, so Sara is so mortified by her relationship with Frank she moves out of town.