You name it so well here- that the more you hold yourself accountable to be understanding and civilized, the more they are liable to use it against you. It becomes just one more way they can exploit and control you.
How wonderful you are realizing that one of the ways to break free from the way this family member holds control over you, is by actually getting really mad! This is a very difficult place to get to for people, especially within the context of family.
I congratulate you for getting here!
Happy Holidays to you!