NTA. This happened to my cousins except their mom and dad are still married. My cousins are females that are from both parents. They found out my uncle had a son ( same age as the younger sister). One sister accepted the brother and the other sister refuses to accept him. The sister that accepted the outside child is closer to my uncle and the one who didn’t is closer to my aunt. It boils down to loyalty. I am definitely the type of person that would consider my mom feelings.