I think it's fine to set a very firm boundary and say if you ever say anything like that again, our relationship is over (or whatever other consequence you can and will enforce). And then you'll have to enforce it if she doesn't change. You don't have to put up with this and you shouldn't.
I can only imagine how you feel. I had to end a friendship for a very similar reason (an extremely close long-time friend needed to be on medication regularly and in therapy to help him manage these meltdowns/exclusions but he decided it wasn't for him anymore and said some things similar to what your sister said and I was forced to tell him that I don't allow anyone to speak to me like that, ever. I ended up blocking him on everything and he's even tried to contact me through other people and a new phone number.
Marriages don’t work when only one person is putting effort in.
You have you’re whole life ahead of you. You don’t need to waste it being tied down to someone who doesn’t put any effort into you.
That's abusive behavior.
She doesn't want to improve.
She's going to get worse.
I think it's fine to set a very firm boundary and say if you ever say anything like that again, our relationship is over (or whatever other consequence you can and will enforce). And then you'll have to enforce it if she doesn't change. You don't have to put up with this and you shouldn't.
I can only imagine how you feel. I had to end a friendship for a very similar reason (an extremely close long-time friend needed to be on medication regularly and in therapy to help him manage these meltdowns/exclusions but he decided it wasn't for him anymore and said some things similar to what your sister said and I was forced to tell him that I don't allow anyone to speak to me like that, ever. I ended up blocking him on everything and he's even tried to contact me through other people and a new phone number.