Well one possibility would be to go there in gender neutral clothes.
Maybe its possible to wear cloth trousers and a nice sweater ?
You might talk about it with your cousin.
Well further I'd echo to possibly stay away from people likely to make a scene.
Maybe its possible to mingle and stay in a small group of likeminded people, shielded from other persons.
Another helpful thing might be to remain polite and non reproachful on that day.
Just letting those people be, maybe telling them to leave you alone, and keeping to your friends.
Maybe a short explanation, that being transgender has biological reasons, cannot be changed and is a recognized condition.
hugs