How to get siblings to accept no?

Who exactly is pressuring you and are they stepping up too?

You can't get anyone to accept anything you say.
You CAN say it, mean it, and stick to it.

Many stroke survivors make good progress, making adjustments as they need.

You mentioned your sister is still indepedent so far?

If so, what help does she need?

It's really not as hard as you're making it out to be, as the word no is a complete sentence and needs no further explanation.
So here are just a few examples below of what to say to your siblings.

NO, I won't be stepping up to care for my sister, but if you feel she needs the help then you're more than welcome to step up and do it.
NO, I really can't care for her at this time.
NO, I'm not really the caregiving type, but again if you're so concerned about her care you are more than welcome to provide it.