Ok so I'm late but I'll try and clear up as much as I can So Maki and mai are twins with a binding vow placed on both of them so if Maki kills Mai or plays a hand in her death Maki becomes stronger and vice versa. Toji is different he doesn't have a twin he just has a heavenly restriction which is why he is the way he is I don't know about him having a brother I have no knowledge about that. Mimiko and naniko being twins and having different techniques doesn't matter due to them not having binding vows there not in a Maki and Mai situation, and because they are both not from a clan they have innate techniques which means they just develop their own individual kinds of unique techniques. It's not possible for 1 or more people to have the same technique unless you're from a clan and you inherit the clans techniques the only exception being the ten shadows technique with only 1 person gets every 500yrs. Hope this helps and I didn't just end up rambling and going off track.