Why do you want those males? The only reason you need a rooster is if you want fertile eggs. Anything else is a personal preference, not a need. I always recommend you keep as few roosters as you can and still meet your goals. That’s not because you are guaranteed problems with more roosters, just that problems are more likely. Sounds like your sister has a strong personal preference.
Many people confuse cockerels and pullets with roosters and hens. Imagine a mixed-sex group of 21 9th graders left alone for weeks with no adult supervision. Now imagine a group of mature responsible adults in that same situation. Would you expect the results to be the same? A flock of adolescent chickens can be hard to watch as they go through puberty and mature into adults. Even if adult chickens are in the flock it can get pretty rough with the adolescents. But a flock of adult chickens is often quite peaceful. There are some restrictions though.
One huge factor in how multiple roosters and hens get long in the flock is room. If the roosters can claim a territory where they can stay away from each other with their harems things are usually OK. One of our members, Centrarchid, has a lot of experience with this. But we are talking about a lot more room than the vast majority of people on this forum have. That’s just not a viable solution for most of us.
Many breeders keep one or two hens with one rooster for the breeding season with no problems of over-mating or barebacked hens. A big factor in that being successful is that the rooster and hens are mature, not adolescents. Some forum members have over-mating and barebacked hen problems with one rooster and over 20 hens, even occasionally with mature flocks. There are so many different factors involved, including the individual personality of the chickens, that there can be no magic ratio where everything works out. We are all unique in our set-ups and management techniques and each flock has its own dynamics.
Every year I hatch and raise chicks with the flock until butcher age for the cockerels, around 5 to 6 months of age usually. Some years I hatch a lot more pullets than cockerels, some years I have a lot more cockerels than pullets. Usually that goes without any real problems. It does get hectic down there but I have a lot of room. The pullets can get away from the cockerels if they need to. I’ve never had a pullet injured or too stressed out from over-mating. I have seen pullets avoid the cockerels sometimes, not always, but I have enough room they can do that. I did have one cockerel kill another cockerel once. I’ve been doing this a lot of years, I’ve had that happen once.
I don’t know how much room you have but I feel fairly confident that you may not have enough room to let them all range together. It does take a lot of room with multiple cockerels or multiple roosters. There is a fairly simple solution, a bachelor pad. Build a pen to house the males. You can leave one cockerel with the pullets if you wish. You can try two cockerels with the pullets if you want to but I wouldn’t go beyond that without a lot of room. If you lock the boys up together with no females to fight over they normally get along pretty well. They will establish a pecking order and all that but it’s usually not much worse than what you see in an all-hen flock. While you can expect some rough times with adolescents that may allow your sister to have her roosters and you to have a peaceful flock, at least after they grow up.