I've been taking a look at Mr. Sinister and his major comic book arcs lately as I have been intrigued with his obession with Cyclops lol. So I was thinking what if he is the Summers ancestor and became obbessed with his descendants because they eventually tranceded to being mutants something he secretly always wanted. Him being am ancestor would also explain how he knows Cyclops has brothers and why he wanted to keep them safe from the legacy virus . I can also see him being Gambit's father , Thus making Cyclops/Havok and Gambit cousins of some sort.