How can I deal with a brother who controls and influences our mother, thereby causing me injustice?

A Muslim must treat people kindly, as Allah Almighty says: "And speak to people good words." Good character elevates its possessor to the rank of one who prays at night and fasts by day, and it is the heaviest thing in the balance. This is further emphasized in dealing with siblings with patience, good character, and supplication for them. If they cut ties or mistreat you, continue to be kind. As for parents, their matter is greater, and the mother's right is even greater.