...ultimately it is your decission. I personally do not like keeping chickens indoors for all of the above mentioned reasons. But one more thing I would like you to consider is the fact that you plan on keeping them in a parrot cage. Parrot cages tend to b taller than they are wide (I will not get in to how I feel about average housing and diet that most people provide to their parrots, just let me say it is akin to keeping a small child in a closet...short of providing an aviary, I will not keep a parrot) to accomodate the length of a parrot's tail, and it's upright body structure. Basically, a parrot cage is a very poor attempt at mimicing living for an avian (flighted) creature, not a ground dwelling one.
If you (or rather, your sister) choose to go the indoor chicken route after doing your research, please reconsider keeping them in a parrot cage. As ground dwellers (and in my opinion flighted birds), they should be kept in an inclsure that is longer than it is tall to give them maximum floor space, which is where they will be spending a lot of time. Also to consider, is your sister willing to put them in chicken diapers and let them walk around the house? If they are going to be staying in an enclosure with a floor space of 40x40 at all times, I would reconsider keeping them indoors.