My mum is a lesbian, one of my half-brothers is gay, and one of my cousins is asexual, so there's a fair bit of LGBT in my family, too, but as far as I know, I'm the only trans* person.
This is just my opinion, but I think it's probably more likely for someone to have LGBT relatives if they are LGBT. It seems logical to me that genetics would play a part. And, of course, if you're closely related to someone who is LGBT, you feel less inclined to stay closeted.