hmmm, this made me think of an acquaintance of mine--well, she's a little more than that, but not quite a confidante, if you know what I mean. Anyway, her ds#1 is repeating kindergarten this year, he turned 6 in August. Not because he did poorly, but he didn't do as well as she and dh would have hoped and they felt that most of the other parents are "doing it" and they feel that "...when in Rome..." Okay, whatever. Her ds#2 and my ds are the same age, June birthdays, four right now. So she's already decided that she'll have him repeat kinder too so that later the boys will just figure, "That's what our family did, repeated kinder 'cause Mom and Dad wanted us to." Okay, again, whatever. I don't see myself choosing that option, but I can kind of see her logic, and it probably won't be as traumatizing for ds#1 in the end that way. So, in that vein, I suppose you could consider it for dd to, if you wanted to go that route, and pose it along the lines of, "This is just what we've chosen to do in our family, wait until you're older to start school." In this way, you'd be able to avoid the whole same class thing.
As a school psychologist, I would agree that late Fall birthday kids, particularly boys, have more trouble. I think it's just that developmentally they're still all revved up [as a general rule, there is always an exception] and aren't quite ready to sit down and do seat work for multiple hours in a day (don't even get me started on what I think about that!). And to that end, I
have considered having ds wait until he's 6 to start school even though he'll be well into 5 when the school year begins next Fall. I can tell you that I can walk into a classroom on day 1 of the school year and choose every single child with a birthday between Sept. 1 and December 3 (our cut-off in this state) within 30 minutes. They just stand out. Of course, this doesn't mean they'll all fail miserably, but boy oh boy can you tell they're the youngins in the room.
As to the siblings in one classroom, most schools do try to separate siblings, and, if possible, cousins. Particularly twins. This is based on the belief that twins will often rely soley on one another to the exclusion of all others and thus won't learn the necessary social skills. I use to be more in favor of this before I had kids. Now I'm not so sure I'm against it. I think it depends on the children in question. While I do believe that it's a wonderful thing for siblings to take comfort in one another while in strange surroundings, I also think that maybe they don't need to be together all hours of every single day, and I would probably make some kind of effort to separate my kids during the school day (in the classroom, not at recess, lunch or other activities), partially because, as much as I love dh, I don't think it's healthy for us to be together 24/7/365. I think we would not then allow ourselves to adequately develop/maintain healthy relationships with others, which is something we all need to do as adults, right? (I guess that could be debatable) So, I would agree that maybe it's not great to have the sibs in the same class, but I don't necessarily think the same grade is so awful.
I hope there is a smidgen of help in that long old essay. Good luck with your decision making!