NTA. You're trying your best, given with what you have to work with. Caring for a child, even a neurotypical child, is difficult. A few questions
INFO: If you went to medical experts and they claimed she was fine, would you be open to seeing 1-2 more specialists if the BO is severe? Are they claiming that it is normal, or that they do not smell anything unusual? Does your husband, brother, sister, whoever else impartial (not you), agree that the smell is unusual and strong? Is your child's diet to blame? Is it possible they have fish-odor syndrome? Are they going through early development/puberty due to eating hormone-heavy food?
A gentle deodorant without aluminum is possible these days, and you can also manage it with essential oils mixed into shea butter and cornstarch if you wanted something super clean and homemade, while gentle on a baby's skin. NT and ND children alike need to learn to maintain good hygiene, or it is another thing to get ostracized from their peers and other unrelated adults when they're young. (I know this from painful experience.)
HOWEVER, your mother/sis may have a point. If this is a chronic condition for Brynn, and one she will have for her life without a cure (fish odor syndrome), your focus on fixing it may provoke body image and esteem issues that you are ironically trying hard to prevent.
A few links related to your concern:
Best of luck, OP, you are a good mom for caring enough to doubt yourself, let alone try so hard for your child. You obviously love her, very much. NTA.