The other mutant family that sticks together is Northstar and Aurora. But they're Canadian so I guess that doesn't count...? Can't believe you forgot one of your favorite characters and her crazy twin sister. Typing that made me realize there are more than 1 set of crazy mutant twins (at least 1 of the twins being crazy).
-Cuckoos: Sharing a hive mind must drive them INSANE. Esme killed Sophie then died helping "Magneto/Xorn" take over Manhattan
-Quicksilver (crazy) & Scarlet Witch (crazier)
- Northstar & Aurora (Multiple Personality Disorder)
-Xavier and Cassandra Nova (Mega-Sentinel crazy) Grant Morrison definitely had them fighting in the womb together.
I'm sure there are others, but those are the the ones that came to mind