I wouldn't blame you if you don't want to go, but, if you have a good relationship with your sister, you might want to figure out if you can either muster the strength to attend the ceremony (maybe not stay any longer than that) or see if there's anything else that you can do to support her at that time. See if she can think of anything else that would make it up to her.
Awake, O north wind, and come, O south wind! Blow upon my garden
I read this to my husband and he said that the wind never did much for him but when I blow upon his garden GIDDYUP!
I wouldn't blame you if you don't want to go, but, if you have a good relationship with your sister, you might want to figure out if you can either muster the strength to attend the ceremony (maybe not stay any longer than that) or see if there's anything else that you can do to support her at that time. See if she can think of anything else that would make it up to her.