Do you have someone who can come with you, stand shoulder to shoulder with you through out and really understand what support you need?
If so go, but have a pre arranged plan with who ever you go with,
That if at any moment no matter what, if the situation becomes overwhelming, you leave, quickly quietly straight away, no hesitation or delays, no discussions.
Have your car parked facing outwards where it cannot get blocked in,
pre warn your sister that you will do your best to stay at the event, but if your sensibilities are that for self preservation, you will leave and she is not to worry about it.
If you have pre planned that you can disengage quickly at any point, it will take some of the pre tension out, give yourself permission to disengage when it suits YOU
Plan with your support, that if you have to go, that you will go and share a bottle of bubble (buy in advance and have on ice in the car) somewhere nice, toast your sister, toast yourself for both doing so well despite toxic parenting.