seating issue - girlfriends of brothers

So - traditionally you've got the "guests of the bride" on the left hand side and the "guests of the groom" on the right hand side right? Which we're doing that's fine, and then I'll walk up the aisle from the back and they'll be looking at our backs when we saw our vows - I'm good with that. However, rather than just have our respective families sat on the front row on each side I'm going to have chairs for them set up facing the opposite direction. There is a small "stage" at the top of the room, on which the registrar will stand and the table with the register will be up there etc - when I say stage - it is literally one step lol, not a big stage! So I'm going to have eight chairs on th right hand side, looking back towards all of the guests, for H2B's parents and their new husband and wife (divorced and remarried) and for his sister and her 3 sons. Then on the left hand side facing back down the aisle as well I was going to have 6 chairs, for my parents, my Grandmother, and my three brothers. The numbers will be slightly odd but will this be OK? The difficulty I have is that one of my brothers has had a girlfriend since the summer, so if they're still together then it'll be a 2 year relationship when we get to the wedding. I've only met her a few times so far as we don't really get together as a family that often and she lives a few hours away so he usually travels to Wokingham to visit her. She's a really nice girl and I will certainly be inviting her to the wedding, but would it be mean to seperate them and for her to take a seat with the main group of my guests, whilst my brother sits on the stage? This would mean my side would have 7 seats. Then my younger brother has a VERY strange love life! Just over a year ago he started "dating" a girl, who is very shy and quiet and when I have seen her at the odd gathering she's barely muttered a word to me, I don't think she's just rude, just a bit awkward in social situations. They spend every waking day together and are off on holiday together this year, and yet they are still adament that they are "just friends".... so I don't really know what the etiquette is there?! I've invited them to the engagement party as a couple (sent the invite "to Rob and Katie") just to test the water - but if I'm going to have my brothers girlfriend on the stage should I have the other brothers "friend who is a girl"???? They literally live and act as a couple - I find it very very strange! For example they had a falling out because she "cheated" on him (all better now).... but how can you cheat on someone if you're not an exclusive couple? I don't get it! It's quite probable that between now and the wedding they'll either stop whatever it is they're doing, or they'll come out and admit that they're together and it won't be such a weird set up. But then I have a 3rd brother who is currently single. So if he gets a girlfriend between now and then, and I'm planning on having the other 2 girlfriends up on the stage with my brothers, then I'd have to allow his girlfriend to do the same, which would mean I'd have 9 people on my side of the stage, a third of whom are not actually family! Hmm... Would it be easier all round if the boys had seats up on the stage with our parents and grandmother and the girlfriends could sit together in the main "audience" (wrong word??) it's only for the ceremony after all - which isn't going to be long - they'll all be sat on the same table for the wedding breakfast!