First, being homosexual is different than acting out homosexual. One can have any number of disordered inclinations and not act on them, nor will them, and one is completely good with God who knows all about these things.
Second, the Catholic Bishops sent out a very careful pastoral letter some years ago in which they said to parents of gay persons that their job could basically be to just love their son or daughter. Gently, gently. They needn't be police.
It's actually not so different from kids dropping away from the church. Love them until they return. Trust God.
This is actually quite a big subject. An all-consuming subject for a homosexual Catholic to be sure. Yet many such, and their families, have the most primitive ideas of what the church teaches. The Catechism is a terrific starting point, always.
I think were I you Emily I would ask about this the next time I was in confession. Maybe go to three different confessors over time. See if one can help you on this one. They are trained at being "pastoral" both to you and to your family member.
Meanwhile your job can be to remind them of your love and that you want them to do nothing that will harm their self-esteem or their life with poor decisions -- no more or less than if they were mainstream. Remind them that dumb is dumb and a major lifestyle change is best done very carefully if at all.
Just some thoughts. Ask your confessor.