Brazilian Butt Lift Using my Sister's Fat?

This is a great question.
The answer is maybe but probably not. The only way this would work is if your sister is an identical twin. Otherwise your body will recognize her fat as foreign and reject it.
At Tri Valley Plastic Surgery we have performed autologous tissue transfer on twins with good results.
I hope this helps.

No, you cannot use other people’s fat. Your body would reject it.About 60% of fat from the transfer is retained and the results are permanent. The size will change with your weight just as the rest of your body does. In other words, the fat acts like it was naturally there. You can get a good idea of the final result after 6 months.

The only person who could be used for a fat transfer is theoretically an identical twin sibling. Otherwise donor fat is not acceptable.

Hi there-
The Brazilian Butt Lift is a fantastic technique that we do often with great success-- but could not be done using your sister's fat (unless she is your identical twin).
This is because your body would re-absorb the fat almost immediately after injection, recognizing it to be from another person.
In patients like yourself, often the best strategy is to have you gain a bit of weight. Once you have enough fat to redistribute to your particular goal, the procedure can be done. After the surgery you can then lose the additional weight, with preservation of your new shape- in other words you will lose weight everywhere, but the shape and proportions achieved at surgery would be maintained.

That will be possible if you have an identical twin. Seek a plastic surgeon who performs hundreds of these procedures, has great reviews and photos, and is an expert in this field.