Quinceñera ( But Have A Twin Brother)

I would just have her brother be her chambelan de honor. That way it will also put him in the spotlight and can continue to share their birthdays

Try this have him have a girl escort him and have him wear a crown not tiara a crown like a king would wear that way hes right there celebrating both of you quinceanera. And also it might sound funny but a quinceanera also first started out for boys and girls but now you only see girls so this would be a night to remember for both of you. And for the father daughter dance you dance with your dad and he dances with your mom, and you could have all damas ,and then your escort, he could have all chambalanes, and then his girl escort, then when you go do dances have them pair up.