If sister gets a breast reduction, can she donate her boob fat so I can get bigger butt?

Fat transplant. No, unless you are an identical twin to your sister, her fat is genetically different enough from you that your body will reject it. Sorry! :-(.
Absolutely NOT! The fat cells would be recognized as foreign and be destroyed by your immune system. This is called graft rejection and could make you very sick.
SORRY NO. Nice thought but it wouldn't work. The fat cells would be recognized as foreign and will be destroyed by your immune system. So it wouldn't work and could even make the recipient sick.
No. Given timing and transfer technical mattersnit is nit a practical undertaking. You would have to be identical twins and surgery would have to be simultaneous. Immunosuppression is not warranted formthis.
Identical twin ? No, can not be done, in breast reduction , breast tissue along with adipose tissue will be removed which cannot be transplanted to another person. Only solid organs are transplanted from one human to another by connecting blood vesals, only exception is blood transfusion. Transfusion is transplantation liquid blood to another person.