MIL annoying 😒

i'm crashing over in my MIL apt since my fiancé & i are renting it once she leaves to dominican republic. my one month old eats every 2-3 hrs and she complains that he eats too much ... 😐 in my opinion he eats like a regular baby should. she offered to make him eat oatmeal and rice with water to make him full and "strong" she says, and i said hell no. hes not ready to eat stuff other than milk. & she keeps nagging me about it, i told my fiancé and hes pissed & also agrees that she's being fuckin annoying. what do you guys think? am i wrong for not introducing my son to oatmeal?