I'm so confused, and I feel weird about this. Idk what to do? (Boy troubles)?

Um, it's really akward talking about this, mostly because it's abnormal for me, and I don't really feel comfortable asking about it, but I need help. Okay, so my best friend has a brother, and I've never thought of him as a pest like the rest of our friends. In fact, there's only two people in our group of friends that can stand to be around him. One of those two is me. Anyway, I've always been able to stand him, I mean, he's not really that bad. He always listens to ME when he doesn't listen to anybody else. We've had our fights, but only cuz I was at my friends house for 15days. It was like I lived there lol. But, I think I'm starting to like him. He's 2 years younger than me in school years. Everytime I go over to their house I get all nervous when I see him, and a couple of nights ago when I was there, I went up to get a drink and stared at his picture on the fridge. (Pictures) I found myself completely caught up in him at her house, I'm just afraid to tell him bcuz he is so much younger, and idk if he would be immature about it. He's kind of a spazz, and immature at times, but a really sweet and sensitive guy too. (I just think that I'm the only one that notices.) Then there's always the fear of me telling his sister, and her reaction. I don't want her to freak out, and I really didn't want anybody to know, but since I don't know what to do, I thought I'd better ask ppl that won't screw me over and tell ppl about it. So, if you've ever had a problem like this before, plz comment. I really need help!