Cat only attacks one person, why and how can I stop it?

I’ve seen other posts on this sub but none of them are quite my situation.

I live with my fiance and her family. I have 3 cats, all female, fixed, around the same age. They are all, normally very docile, loving cats.

One particular cat, is usually very loving, often to an annoying degree. But every once in a while she’ll attack my fiance’s mom BADLY, it seems completely unprompted.

Usually the answer is that the cat is redirected aggression or is in a fear state, but this doesnt seem to be the case, because I or any one else can grab the cat and hold her in the middle of an attack and she doesnt not attack any of us, nor even accidentally scratch us. It is purely random acts of violence.

It happens about once a month, seemingly at random.

We’re at the point now where there doesnt seem to be an option except to put her up for adoption.

Does anybody have any experience with this, or can offer any advice? We’re at our wits end here.