Is it possible to use donor fat, like from a family member, for a BBL if you don't have sufficient?

 You cannot use someone else's fat for any procedure whether it is cosmetic or reconstructive. You're fat and tissues are unique to you. The body will reject someone else's fat as foreign.

Unfortunately that is not an option. Your body would reject it. There certainly are options for gaining enough weight for a BBL. My recommendation is that you find an ASPS board certified surgeon in your area who specializes in the BBL, and set up an in person consult. An in person consult with an ASPS board certified surgeon is the only way to determine the best route for your body and your goals. On some occasions patients who are devoid of the fat cells necessary for the BBL are asked to gain weight. This however is not indicative of survival rate or post-surgical success metrics. I have had certain patients gain weight so that we had enough fat to transfer. It is all about designing the perfect plan for You.Good luck, please let us know how it goes.-Dr. Constantino Mendieta

Unfortunately not.  Fat cells have markers on them that would tell the body that your family members fat cells are foreign.  Your immune system would reject them.  It would only work if you use the fat from an identical twin theoretically.  Some people can gain enough weight to make a BBL possible.  Discuss whether this is possible with your doctor. Best wishes.