how do i tell my sister her breath smells really bad

hey. i really want to tell my sister that she has bad breath but i really dont know a way to go about it without upsetting her.

to put it bluntly, her breath is often hot and smells like, im not gonna lie, straight up poo its THAT bad.
my sisters breath hasnt always smelt bad. it mainly started to smell really bad when we were at the end of primary school. she used to get occasional tonsil stones that would land on me while she was speaking and it would bother me. and if u know me personally, one thing i cannot stand is tonsil stones. i dont play about my dental health and tonsil stones gross me out. there was a time where we were in a shop and she was speaking to me and a tonsil stones of hers landed on my lip. being about 11 years old, immature and socially unaware i yelled at her in front of everyone at the shop. telling her to start brushing her teeth. she got embarrassed and i felt so sorry especially after when my mum started to yell at me telling me how rude i was for that. i regret embarrassing her about it and i haven’t brought up her bad breath since. now that we’re going into college next year, its still the same problem with her breath

and because she’s sensitive, people dont often bring it up to her. i remember when one of my friends from school told her that in the morning her breath sometimes smells bad. i could tell my friend was being genuine, it was almost like she was walking on egg shells talking about it but my sister got defensive, telling her that she was being rude and refused to talk after that. when we got home my sister spoke to me about it, saying mean things about my friend for just trying to help.

i feel sorry cus there are times where she talks and i cover my mouth or i back up slightly, and she looks sad afterwards.

i want to use this summer holiday as a chance to tell her how i feel. we’re going to a new school next year and im worried people from school are going to take notice to her bad breath and maybe pick on her for it.

what do i do? how do i tell my twin sister😭😭?