You have a tricky one OP as either way now, your daughter could resent the decision to override her first choice (she'll get over it, she's still 11) or your family's impact will be huge on her 1st choice. Don't forget the toing and froing on top you'll need to do for concerts, plays, sports, parents meetings etc.
She's also been shown an opportunity, gained the place, but may not be able to now have (I know, your school pressured you into applying). Dependent on what the guidelines when you started this, it will be a challenge.
My DD walks out at 7am and returns at 7/8pm most nights due to extra curricular. It's tough, but she's done her homework along the way most days and we eat together...but I agree it would not suit most, and she's an only child. Your DS sounds like he's going to miss his sister what ever option you take that's not his school), as she may seek her independence and force family dynamic changes regardless, and quicker than you anticipated.
Boys - her motives may be a little suspect (to you), but if she's already spent junior days in a girls only, this is perfectly logical why she'd want a change. We had the same scenario...so far, so good, and can dilute the negative aspects of girls...
Bags - the final note of caution, depending on what she does. This is mentioned a lot, but until you see the volume on certain days, (school, sports, instrument, swimming to name a few), factor this in to the travel arrangements, especially if some of it is by foot as you anticipate as she gets older.