How could Chara take over the skeleton brothers?
Hmm, lets see, there is one thing a lot of characters have in common in that story. And that’s DT.
It obviously came from somewhere, and where you can find in sufficient quantity is a human soul.
Before you think Blaster Gaster is a bad guy by experimenting on an helpless child, well that’s not true, since Chara was the one volunteering to do it as they wanted to help save the monster, and actually liked to tease Gaster when he was working or pulling pranks on him (They actually carved a hole on top of his skull one time, it’s visible on his semi blaster form).They were friends, kind of, since they won’t hesitate to attack each other for fun.
Anyway, back on the subject, Chara can control the brother because The DT used for creating them was Chara’s determination (Later on the DT used is coming from other kids souls). but Chara’s DT will not be enough to control them if they are doing fine and in their right mind, but if they are knocked unconscious or turn feral they become easy to control.
Gaster can do the same thing as Chara but kind at anytime (except if they are already being controled) because they all share a part of his soul.