The question came up recently of what the purpose or significance was between brother and sister flights. I throw out the phrase quite a bit, so I thought I'd do a formal post on the topic.
Squadrons are set up so that each stairwell accesses two dorms per floor, on each of the two stories. Those two dorms share a common foyer, which includes (at least in our squadron) a drinking fountain (that no one uses), a window, and the doors to each of the dorms at a diagonal to each other. The two flights occupying these dorms are brother/sister flights or brother/brother flights. I would assume it'd be pretty rare to have a sister/sister flight, since male trainees generally outnumber females.
Brother/sister flights arrive at BMT on the same day and graduate on the same day. Due to the close proximity of the dorms, they share the same MTI team and their MTIs work closely together. Your MTI and your brother/sister flight's MTI has access to your dorm on the dormitory access roster, so you should expect them to walk in unannounced at any given time (within the duty day). Our MTIs would occasionally share responsibilities; if one had to run an errand or attend to a personal matter, the other would watch both flights. We got very used to seeing our brother flight's MTI in our dorm.
You'll spend a significant amount of time with your brother/sister flight, as you'll do all of your training together. On second thought, you'll do almost everything together. All of your hands-on training, all of your classroom training, all of it will be done together. Your brother/sister flight is your other half at BMT. You'll learn each other's names and swap advice with your counterparts. I started referring to my brother flight Dorm Chief as "My B-9 Boyfriend" because we had our nightly meet-ups to head down to accountability.
Initially, your MTIs will encourage healthy competition between you and your brother/sister flight. It wasn't uncommon for our MTI to build us up as a flight by telling us how much better we were than those "smelly males" across the way, and how much better females do at drill. Ultimately, you'll be reminded by older trainees and even your MTIs that you can't take this spirit of competition too seriously, as you'll have to work hand in hand with your brother/sister flight at BEAST. Your success out there depends on it.
Even after BMT is over, you'll still keep in touch with your brother/sister flight, just as much as your own flight. You're likely to find yourself at tech school with some of them, getting to know them even better than you did at BMT. We had at least two relationships come of that closeness. Most of us are in the operational Air Force now, and we still communicate on an almost daily basis via a Facebook group.
Here's to your new Air Force family!