Not bad, the artstyle is good and MC power seems to be focused on a single spell that only he can use in a different way than others.
In exchange for it having a higher cap, or no limit at all, he can't use other spells, balancing it.
Saying the sister is stupid is wrong, as her whole life was the prison and she is a true child, not unlike MC that have a past life knowledge, for her their life is normal.
Is fairly clear MC and his family are politicla hostages, most likely the mother married the bastard ruler for a peace treaty, the collar must be a magic seal, that means that keeping inside the prison is a choice as they could just use the collars around the castle, is not for protection because if it was their "father" wouldn't have put a endgame boss as guard at the slightest perceived escape attempt, not even a alarm bell, outright super murder, speaking of that I hope that together with mastery there are levels and MC now got a massive amount of exp.
I must say that the reincarnation gimmick is stupid and the reason too.
MC was playing a new game and he loved it, marathoned it to the end and gone to sleep, no sign of exhausting himself to death nor a scene with him dying for some reason like a bloody nose, a stroke in the bed, slipping down the stairs, he woke up dead and reincarnated in a game he just played, unlike other similar stories where the MC loved the game to bits and played every possible scenario for months/years, very weak.