I had to do this recently with one of my sisters.

She's so self-centered to the point where I just can't stand her, and she keeps crossing boundaries that I have repeatedly set.

Turns out, my mother begrudgingly admitted "She's not malicious, she's just mind-numbingly stupid." I observed quietly for some time, and.... yeah. She's just stupid. That's the only way I can't be mad at her, so that's what is keeping the peace between us.