Brazilian butt lift for petite 5'2 101 pound girl. (Photos)

This question of weight gain as it relates to BBL is fast becoming one of the most commonly posted questions in the RealSelf community. While it is true BBL is a weight sensitive procedure (in the sense that you need fat to achieve your result), it is never a good idea to pursue short term changes for surgery.
While there is no absolute number (lbs.) in terms of weight for undergoing liposuction with autologous fat transfer, the optimal candidate has reached a healthy and stable weight (preferably with a BMI less than 30, which is associated with a lower incidence of complications).
Potential candidates should also have adequate fat reserves for transfer. This becomes especially important when one considers the anticipated rate of fat resorption and graft loss. I typically recommened patients aim for at least 500 cc transferred per cheek. This would amount to 1 liter of fat (not lipoaspirate) after processing. Candidates with inadequate fat reserves are likely to be disappointed with their ultimate volume once healing is complete. 
With regards to your specific question, I would not change your weight for the sake of surgery. If you have reached a healthy stable weight for you, the focus should be on maintaining. I have found that patients who purposely gain weight before surgery will either struggle to lose weight post-op or will lose it in the grafted area post-operatively. Remember grafted fat will behave just as if it had never been grafted. If you gain weight for transfer, and lose it, you may also lose your result.Based upon your photos, you are in excellent shape. However, you do not have a significant excess of fat available for transfer. You might want to consider implants (although this will often require some measure of fat for transfer and camouflage).
As always, discuss your concerns with a board certified plastic surgeon (ABPS)

Hello dear!Thanks for the question and photos provided.Based on your photos it's a fact that you should gain some weight to performance a BBL, because you are skinny.I recommend you to make an appointment with a board certified plastic surgeon to talk about your concerns and goals.Good luck :)