I would be annoyed.
Depends on what she actually said that was making fun of her?
@Laura she was fake crying and teasing her for crying. I was mainly annoyed because crying is the onlu way for her to communicate since she's a baby. She can't exactly say "I'm hungry" yet. Why make fun of her for communicating that she needs something?
My baby brother is 15 and it's like my son is his chance to be a big brother 😅 He's not mean-spirited when he "makes fun of" my son for crying (when he was younger), he's just being a butthead. I agree with what others have said; it sounds like she could've fed the baby a bottle and you've killed 2 birds with 1 stone.
Personally, no. I would have given the sister the bottle to feed the baby.