I was about to post something similar
Your sister will feel awful about your Mums condition…. Whatever her faults, she didn’t want to try and give your Mum COVID.
Do what you need to go to get your Mum straight as well as taking care of your own health
If I were you I wouldn’t even bring up the Xmas incident with anybody in the future…. Just let it drift under the bridge and celebrate (in the future) that you are happy, healthy and together again…