My brother is a young 17 year old who dresses as a girl..?

My brother came out of the closet a few years ago... he believes he's a girl trapped in a boys body.. i accept that. I come from a Mexican family and my dad is doing his best to get used to it, but his side of the family talk a lot of ****. They make indirect comments but you know what they are aiming at. I could care less, but my dad.. my poor dad... it feels as they are taking a hit at my dads masculinity because its his son. A few days ago my uncle invited my dad to go over his house for a birthday... n the whole night my uncle kept talking about how his son being "muy hombresito" or very much a man cuz he has had girlfriends and has a girlfriend.. he just made comments like look at MY SON he gets girls He's a such a stallion he's a man... n then my other uncle said.. like father like son. he said all that in front of my dad... he was hurt ...upset. Idk what to do. I get so mad because that's such a low blow. And my dad.. :( what should I do?