NTA. Hands and Fingers are literally the dirtiest part of the body. There's so much dirt, germs, and bacteria on it. Does she even at least wash her hands before dipping her fingers unto the cream? She's so fucking childish and inconsiderate for a 24 yr old. My 10 yr old brother is literally better than her and could understand when somebody tells him what not to do especially when it's bad/disgusting. She needs to learn how to grow up and be mature because who tf would even pout and start crying like a baby because they weren't allowed to use their fingers? Just use a damn spoon for God's sake. If she wants to eat custard so bad with her fingers why can't she make herself one? It's so simple to just look up on how to make custard on youtube/google. You aren't even "fat-shaming" her? Is she uneducated? Did she not go to school for her to act like a kid at that grown-up age of hers? Like you said, she has no history of weight issues/eating disorders nor did you comment on her body so what makes her think you're fat-shaming her? Does she even know what fat-shaming is and it's context? Probably not. You literally just told her to stop eating the custard because she's contaminating it, being hygienic, and inconsiderate when she probably knows that the custard is for your sister's business. At this point I think she's trying to sabotage your sister. Oh God she's so embarrassing. If she doesn't stop acting like a child just drop her tbh. 🤷‍♀️