I really want a guy i (almost) know is gay

I have a very good friend of the same sex who is a few years older than me. She is very close to her brother Danny - before I ever met him, she told me that he is bisexual. She is Brazilian and she was telling me about her relationship with her mother - she happened to tell me that her mother disapproves of her brother (Danny)'s bisexuality - she said her mother doesn't understand it and condemns it and thinks it is only a phase. I can't remember exactly what she said because we were both drinking, but how she phrased it it sounded more like he was homosexual than bisexual.

Soon after that I met Danny - he is very good looking, very sweet and personable, lovely really. After I met him my friend (his sister) told me that he said I am beautiful - after that I couldn't help but think of him in a different, more sexual light.

Since then, Danny and I have become friends - he has looked after me in situations where he could have taken advantage, but didn;t. I have hinted to my friend his sister that I find him attractive, and she's ignored it - I feel like if he was straight she'd be more like, hey, I'll hook you guys up - but she hasn't been. On the other hand, both of their parents are dead, so they are very close and maybe she doesn't want to jeopardize that.

Realistically, I believe this guy is gay - from the way his sister acts about him, from the way he is (never talking about sex or girls) and from the way he relates to me (he said he thinks i am beautiful, but he's never tried to get with me). But despite that, I can't stop thinking about him. I feel like I'd be crossing a line if I asked my friend (his sister) if he is gay - I feel instinctively that she wouldn't like me to fancy him. Is it normal that I basically know this guy is gay but keep convincing myself otherwise and thinking about him?