I don't know what the mom is complaining about. She told OP to deal with, and OP did. She wanted her boyfriend's kids to use that space. Now that OP is gone, they can use all that space. If there are other consequences she failed to consider before forcing OP to share her space and things, that's her problem.
Stand your ground ur mother was fine with u having the basement to your self sounds like her new boyfriend is turning your mother against u, wait they will be kick out soon enough then u can change the locks on the basement maybe make a door to your basement