one of my 2 year old cats just keeps being aggressive to my 1 1/2 year old cat, and i don't know where to go from here..

(cross-posted from r/cats as i received basically no help)

so, i have five cats—all fixed. angela, who is 3, and the mother of tortellini and toralei, who are 2. then there's juniper, who's 1.5, and cheeto puff, who is about 5 months i think? (he's a rescue, so i'm not quite sure, but less than a year definitely). i adopted juniper when he was about 6 months, and got him neutered immediately. well, tortellini hadn't minded him.. at first. then one day she just suddenly HATED him, and has been aggressive and antagonistic toward him ever since. no amount of time, or training attempts, or reprimands, or praise, has ever cut out this behavior, and i don't know why she behaves this way toward him. he's a VERY sweet cat—the sweetest and most well-behaved out of all of them. he's never once given tortellini a reason to behave this way, and i've seen her go out of her way to be hateful to him! :( occasionally she will be nasty to the other cats too, but juniper gets the brunt of it and she has actually hurt him. my thing is, though, she's never known a single day as an only cat. and she has no problems with cheeto at all, so it's not a new cat thing, and it's not a male cat thing.. and i realize the only truly responsible option here is to rehome her, but i have tried SO HARD to find a home for her, and no luck at all. i don't want to just put her outside because she's been an indoor cat all her life and that's ESPECIALLY unsafe in the area i live in. but i don't know if taking her to a shelter is the kindest option either, because she could very well end up in a bad home and that terrifies me because i don't want that for her—obviously. what is my best option here? i guess i need some direction and outside opinion, preferably from cat owners who have dealt with this but honestly, any advice is welcome. thank you! 🥹