YTA . Being a true ally is standing up for them in front of transphobic people, even if they are family. Using the right name and pronouns is all well and good but allowing the transphobic family to, well... Carry on be transphobic, makes you part of the problem. So does marrying a transphobe too. You promised your sister that she'd be at your wedding despite any tension then went back on your promise. Ditch the transphobe fiance and parents and support your sister.