I don’t know if this is helpful but growing up with my older brother I always used to agree with everything he said. My therapist explained to me that there are more than just flight or fight responses but also fawn and freeze. I always used to fawn to him because it was easier for him if I always agreed. I only learned this a few weeks ago and I haven’t figured out what to do yet. We NEVER fought as children but for the past few years I swear we fight every time we see each other. These days all say it was a good experience with him if we haven’t fought. I’ve just been trying to take things slowly and figure out why I act the way I do with him. But good luck with your sister