From my perspective it sounds like you consider your sister's life as more important to you than your relationship with her. You are not the asshole but one of the assholes here, an ethical asshole as far as I can say see, the other being your sister who recklessly ignored your advice and then had the audacity to feel sorry for herself in front of the one person who repeatedly told her to get tested.

You now have a choice and it's gonna be hard. If your sister isn't going to make it through you should probably swallow your pride (just remember you were right and put it aside) but if she will be okay and this is a life lesson she needs to learn the hard way, you better leave her to her own devices.

You have every right to not forgive your sister for being careless with her own health and life. It's her choice because it's her body but your feelings on it are perfectly valid and you're entitled to them.