How do I (M26) deal with a difficult relative(F32)?

This is a bit of a rant. I (M26) moved to a different state recently and am getting ready to go home for xmas next week.

My sister (F32)for the past month has been suggested i wear a santa costume for her kids (toddlers). I said i didnt really feel like doing it but of course my words alwqys fall on deaf ears. I called yesterday to check up and the first thing she said was that i was “lucky” that i she couldnt find a costume at a reasonable price because i was going to wear it. I said i never committed to it and that she never asked. I didnt press it too much cause it wasnt happening.

I was working during our call so i wasnt as involved. Then we added our cousin to the call and my sister started showing her all the pajamas we are going to wear for the pictures and how she bought them for all of us the other day. She knows thats not something i enjoy doing, and if shed ask id consider it, but it got me angry because my voice goes on deaf ears alot with my parents and sister. It almost makes me want to cancel my flight and not go at all.

My sister has always been hardheaded and has a bad temper. It has improved through the years but when she really wants to do something she doesnt take if or buts and expects it to be done regardless of whether we want to do it or not. The family (me included) just give in to her demands but im honestly tired of just giving in to her demands. My parents raised me to walk on eggshells for her and still expect me to do it and ive always gotten the short end of the stick with my parents because of it. Its sad to say but ive been much happier away from family these past few months than ive ever been. Even my therapist is worried about me going back because she knows the abuse (her term not mine) that i went through.

My plan is to still go and if she presents me with the pajamas im just gonna straight ip say no and if it becomes a problem ill walk out if my parents try to be the bigger person and shut up i may just get the next ticket back home. Alot of this may be from recent anger.

How do you deal with difficult relative?

Tl;DR sister bough pjamas and santa suit knowing that i wouldnt want to wear it. Me and my family are tired walking in eggshells for her and im considering walking iut in xmas if this becomes an issue.