I know the origin of both sinister have changed since their creation. Since original the original writer Chris Claremont intend to have them as two sides of the same person, both engineered beings. Someone in the shadows used them as pawns to farther his own ends. CC left the X-men before he was able to reveal his full plan, although he modified it somewhat in X-Men: The End series he wrote. Still it would be cool for the movie to touch on the connection and that it's deep that just having Gambit as a hireling of Sinister.