My neighbor cheated with his partner’s sister, and now lives with the sister. The most aggravating part of it is that he has kids with both. Every other week his ex has to come by to drop off her kids at her sister’s house since they share custody. From the kids perspective, their sibling is also their cousin.

Her ex’s new girlfriend.

Someone who’s already cheating on a partner of their own. The relationship is likely to be a self fulfilling prophecy.