I did similar to this for a Look Good Feel Better charity thing that my wife organised.
I took general documentary stuff while they were getting their makeup done (this was just a makeup thing) then did a formal headshot style portrait in the spare bedroom which I'd "converted" into a studio for the afternoon.
The first thing my wife did was to arrange people to come at half hour intervals so that people weren't sat around being bored. People stayed and chatted, but it took about 30 minutes per person for my wife to do their makeup. If everyone turns up at the same time, it could be a *long* wait for the last person.
Photographically, that meant I was concentrating on one person at once (less stressy). Once they'd been "finished", I took them to another room to get their portrait done. Because it was a headshot (we had no space for anything else and it was raining outside) I had silly sunglasses and headscarves for people to play with. Glasses don't always have to cover the eyes (the over the rims look is in this year I tell you!) and headscarves don't have to cover hair. I had plenty of hands in shot too, with hands framing faces by cupping the chin, or making that L shape framing chin and side of head. I was after a single image as I was giving away one print at the end of each session. I also had to work fast to take the portrait, process it, print it, then get downstairs to documentarise the next person in the line. It was exhausting!
Ultimately though it's all about rapport. Some of the ladies that came weren't confident people at all, so it was down to me (a strange male) to try and get the best out of them. Out of the ten or fifteen ladies that day, there were a couple that just looked uncomfortable regardless of my attempts to try and relax them. If you get that, just put it behind you and move on. Some of them may be getting "made up" for the first time in their lives and may feel very uncomfortable because they look so different. Encourage them, tell them how fab they look and develop a few emergency lines that might get a smile or relax the tension. Remember that they are normal people in an unfamiliar place, with strange stuff on their face and their hair in an unusual state, facing a camera with softboxes/brollies and some strange bloke. They're not professional models used to this, they don't know what to do (apart from look fearful) so it's entirely down to you to get them to feel comfortable.
Set your camera & lighting up in advance. Use your sister as a practise model to make sure it's all good, and put a masking tape X on the carpet for people to stand or (better but we didn't have space) a chair (with a spare cushion to raise short people) so that different heights are less pronounced. A chair also puts you above the subject which can be more flattering when the subjkect is looking up. I found that time evaporated once things got going and fiddling with lights was something I was glad I'd sorted before I'd started. You also put across a more competent image if you're not taking a load of test shots and adjusting things
Good luck! Hope it all goes well. We managed to raised about £300 for LGFB over one afternoon/evening which was really nice. It was fun, but exhausting.
