Not to dwell on a particularly dark tangent, but why do folks think Cyke and Sinister's canonical early years cannot be conceivably integrated in an all-age cinematic universe? There isn't a single parent/educator out there who would believe for one measly second Sinister didn't do even more horrific things to that poor kid than what was shown.
Honestly, the only way to make it work would be for Sinister to be presented as a genuinely cool evil mad scientist foster dad.