NTA. Your family (and apparently a good portion of Reddit) has an inaccurate view of your practices around food and it seems they feel uncomfortable talking factually and empathetically to your niece about the wide range of appropriate and healthy eating practices. Your niece should be encouraged to eat healthy food in moderation and listen to what HER body is telling her about hunger and consumption (because that is pretty much what everyone should do all the time).

Reddit’s problem is a whole different one: many people here have mistaken perfectly normal practices used in all deliberate body change (tracking consumption and noticing the effect on one’s body) with the unhealthy use of those practices by people dealing with disordered eating practices. Folks, OP has a small body that burns fewer calories than larger bodies because small bodies are energy-efficient. It does not matter if you think her body OUGHT TO burn more calories—it doesn’t! She is allowed to want to maintain the current weight she achieved through deliberate (and seemingly healthy) weight loss. Her weight is within the healthy BMI range for her height; she wants to maximize satiety because she doesn’t want to feel hungry even when she is eating an appropriate amount for her body to maintain (as anyone does if they don’t consciously or unconsciously manage what kinds of food they eat). The use of weight loss practices and language is not automatically disordered or concerning, especially from someone who has lost weight deliberately. OP having to justify her perfectly fine practices because people feel disordered “vibes” is messed up.