Etiquette?

So I've only really smoked with the same people the whole time since I've started smoking. Because of this I don't really know what's appropriate when I am around other smokers.

I like to hang out at my brother's apartment sometimes, and I'm not sure if it's okay to accept when they offer to pass the bong to me. Like he, his roommate, or one of their friends is usually smoking when I'm over there, and sometimes the bong will be passed around. I've never helped them pay for weed (although I did share alcohol before) so should I accept the offer? I've both declined and accepted in the past. But I've never outright asked to smoke their weed.

Also, the weed I've been getting lately is like a higher mid grade and they smoke a lot nicer stuff, so I'd like to start buying from my brother's dealer. His weed is a Lot better. Should I just ask my brother for his number? Or should I wait until the next time his dealer is over? I've tried getting my brother to hook me up in the past but either his dealer was currently not around or was out of weed. The other day I bought half a g from my brother's roommate, but he doesn't really deal so I can't ask him for much more than that.

At some point I would like to bring some of my own weed over to his place and let them smoke too (their friends seem to do this pretty often). But since they smoke a lot better weed than I do I'm kind of embarrassed to do this. Should I anyway to avoid coming across as a mooch? Or should I just wait until I get better weed?

My girlfriend and I also want to smoke them out pretty soon once we move into our apartment and get the bong that we're wanting. I've never really hung out with someone with the sole purpose being to smoke them out like that. So I'm not really sure what you're supposed to do. Should I just ask them to come hang out or should I make a point in telling them that that's what I want to do?
And also, is it acceptable to smoke someone out with lower quality weed than they smoke on a regular basis? Should I try to get a better dealer first?

This one guy I know (my ex actually) has pretty good stuff, but my girlfriend and I recently kind of had a falling out with him. So we don't really hang out anymore... But I think he would sell to me, I just think he might also try to rip me off. Around here pretty much a gram of anything is $20, so how much would larger quantities cost do you think? Like say a half ounce or an ounce?
And is it okay to ask to see what you're buying weighed? Even if it's from a friend?

Sorry that's a lot I know, feel free to just answer part of it.

 

Thanks for the help guys.
And here, 20 a g can get you really nice stuff like blue dream and bubble gum, or it can get you totally crappy stuff full of seeds.

My apartment is like 15-20 minutes from my brother's and neither he nor his roommates have a car. So we'd have to go pick them up. So I can't just say "come over whenever" like that, I'd have to specifically plan to have him come over.
But I think my roommates and I are going to have a house warming/welcome back to school party with our friends so I could invite my brother to that I guess. I'd just let him and his roommate smoke and drink whatever they wanted for free.

 

Haha we manage to have pretty fun parties anyway.
I'm not trying to win/win, I'm trying to be polite to my brother and his roommate. But that's a good idea for the future, thanks.