Breakups suck. At best, you never have to see each other again, proceeding upon your separate paths with the personal growth and learning experience behind you. At worst, your ex is part of your family forever and you have no excuse not to be consistently reminded of your past.
A recent Reddit thread was dedicated to the ultra-specific, ultra-awful worst case scenario: when an ex marries into the family (through someone else, of course). One guy's story went viral with its lesson on how kids raise the stakes.
TaylorSchwiftyAMA's story gives your breakup a run for its money:
I dated a woman for a couple of months many years ago, then broke up with her. However, she adored my family, so she continued to hang around with them - parents, brothers, etc. Fast forward 10 years, and she's dating my younger brother, and I have to invite them TO MY WEDDING. Fast forward another year or two, and THEY get married... so now my ex-GF is my sister-in-law.