I'm sorry the virus has impacted you and your family, and I hope things get better for you all. Unfortunately, uncertainty isn't kind to everyone, but things should look up and get better, hopefully before your exam.
For now I would recommend completely isolating from your Mum, obviously, she needs help but it would be best to keep her in bed and leave food outside the door (maybe a bit extreme but the safest measure). I can understand psychologically this feels odd but it's done out of endearment and care for her and yourself. Use electronics or talk from 2 metres away. Keep an eye on her though. If she does show signs in difficulty breathing don't be cautious in calling an ambulance. If you can make a face-masked for yourself, her and your sibling.
I wish you all the best. Statistically, as long as your Mum isn't 70+ and has no underlying health issues there is a very high chance she is going to be fine and her natural immunity will fight it. It may be a hard couple weeks, but I would recommend trying not to stess as you're doing all you can right now. In the unlikely case that she does need to go to hospital I would certainly get in contact with your uni. Furthermore, have they not implemented a policy such as you can't fall below your average from the previous year/term etc? A lot of unis have put measures in place for situations like this.