It sounds like you are an amazing sibling to your sister. Yes, there is a lot of stress. I get it! I was a caregiver to my mom, though only part time, have a full time job and family of my own, so I understand where you are coming from. There are definitely not enough hours in a day to get everything done. You just have to do what is most important.
You cannot be expected to do this all on your own. Do you have siblings or friends or a community that can help you with your family or help your sister's family?
As for your mother telling you regularly to "suck it up"? I don't know your mother, her age, condition, or ability, but that doesn't sound very sympathetic or helpful! She should be so proud of you!
Can't she drop everything and get on a plane/drive to help as well instead of criticizing you? Maybe not........... In any case, she should be supporting you with the fact that you have all of this on your shoulders. I'm sure you are doing the best that you can, and are appreciated by your sister and her family.
So just take a deep breath and do the best you can. Get rest and help when you can, and just work according to priorities. Somethings will suffer, yes, (like a messy house) but that's what you have to do.
Good luck and hope for the best for your sister.
Punky