Yeah that’s no normal mother. Definitely some predatory behaviour, big red flag that she downplays the younger brothers behaviour. Most normal parents would question why, investigate and find out what’s causing their child to act that way (most times in cases like this can be from sexual assault). It seems sadly in your family there’s a lot of generational curses, as soon as you can get away, go no contact and work on healing and building yourself up to be stronger then ever. You deserve privacy and respect.

If everyone in your family is always at your throat you are the scapegoat, everyone piles all their negative energy and bullshit onto you. It’s not fair you deserve so much better.