This has bothered me from day one. I can see Xavier and the X-Men attempting to play nice with Magneto, Mystique, Exodus, the Hellfire Club, even Sabertooth as sociopaths have their uses. And, it's a stretch, but I can imagine the X-Men attempting some sort of an alliance with Apocalypse.
BUT NOT SINISTER.
I was reading the X-Men when Mr. Sinister was first introduced and he was like the ultimate bogey man! He was this cruel insidious presence whose reach extended all the way back to before X-Men #1. He massively fucked with Cyclops' life. He's the kind of guy who would saw off a child's skull and attach electrodes to his brain just to see what would happen. The only person I can compare him to is Josef Mengele the Nazi's angel of death. Sinister is a vile creature and I can't imagine the X-Men tolerating his presence on Genosha.
(Is Sinister even a mutant, anyways?)