I could ask this same question. I have a 12 yo Daughter and 6 year old Son and will be having a baby shortly. well, we only have two rooms for the kids so the older two are going to have to share. I can't see putting either with a baby since they are in school and need their sleep. I couldn't get either of them to sleep in a crib in a separate room as babies so i'm going to do my darnedest to get THIS baby to do it! LOL anyway, with me in and out of a room all night with an infant I would hate my older kids to be woken up. So, is 12 and almost 7 girl and boy too old to share a room?
I don't see why it would be. Obviously the 12 year old girl would like her space and will eventually get it back but for a while it shouldn't be an issue....and i dont' think it should be for you eitehr. At 7 and 5 they are still young and if they are close then it's all the better.
I think some of the people that would protest this are actually putting some sort of sexual connection to the question as if the siblings will just naturally be attracted to each other or something b/c they sleep in the same bed or room! If you decide to make them separate be sure you don't imply anything to them about it being bad to sleep togehter (as siblings) but rather that it's better for them in that they would get a better nights sleep if they weren't fighting for space or covers or whatnot.
I'm not sure why your 7 year old would be more dependant on sleeping with his younger sister but i'd highly doubt it's for any reason that you'd frown upon. 7 isn't all that old and he could still be struggling with some sort of nighttime fear or whatever. I'd think that chances are, as he matures, he will start to out grow that but whose to say. Ask him why he likes to go to sleep with his sister and make sure there isn't some fear you could help him with.
Otherwise I wouldn't worry about it right now. i think siblings will stay close to each other when they are able to rmember times like this when they were close. They'll probably be at each others throats before you want them to be! LOL maybe they will remember those old days when they were so close and forgive each other for being jerks when they are teens! LOLOLOL