Not necessarily. It's a natural male instinct to be attracted to shown skin on any female body. They can't help it, it's like a bug to a beam of light.
If he's your brother's best friend, it would also be quite hard to tell if he likes you as he doesn't want your brother to disown him and end a friendship. Plus, your older brother would be fairly protective of you; guys around the teenage age mark talk about a lot of things with their mates when they like a girl i.e. how big her breasts looked, when you hooked up with her what noises she made etc. Pretty disgusting stuff. So your brother would be concerned with him, thinking he wanted to do all of that stuff with you. It would make things pretty awkward between them.
In guy terms, it's basically nearly almost illegal to date your best friend's sister. If your brother's best friend does like you, he's going to have to earn his share of respect when he tells your brother. and of course. big brothers give permission.
Pretty sure most guys are going to look at your breasts whether he likes you or not. That's just what they do. They like to look at our asses and I'm told that at least some of them check for panty lines to try to imagine what kind of panties we're wearing too.