So if you have a twin and your partner constantly jokes about wanting to sleep with your twin, and you expressed how much that bothers you but your partner keeps at it. Should you break up?
If they told you that its even another reason to leave and find someone new. That is kind of creepy that they are not only fantasizing about your sibling, but disrespecting your family to your face as well. I would want to beat them up honestly. They have no love or respect for you as a person.
It sounds like the partner really wants this and is trying to grind the twin down. I don't think this is ever going to go away, so yes, it probably should be a reason to end the relationship.
Maybe it's not an attraction to your twin as much as it's a 3-some with twins fantasy thing. Idk? If you're not comfortable with her constantly mentioning it then yeah break it off with her.