I want to help care for mom, but not every weekend
Before I begin, I would like to ask you to turn your eyes to your mother. While I am pretty sure your mother lived her life independently, now she has to lose her independence and rely on you two, her daughters, for basically everything. The parent-child role reversal is a humbling experience for your mother.
Your decision to take care of your mother is very loving. Taking care of her encompasses two very well-defined areas, and they are very different in nature: One is tending to her daily needs, and the other is spending meaningful time with the woman who gave you life and raised you.
Perhaps some of the daily tasks of preparing food, bathing, dressing, etc. could be done occasionally by aides who provide in-home care services (both during the week and on weekends). If you are able to get some help with the chores occasionally, both you and your sister can get a needed break, and the quality time you spend with your mother will make her feel loved and cared for.
And remember that you have to take care of yourself. Although there may be difficult days, I feel certain you will cherish this time spent with your mother in the years to come. It all comes down to love, which “bears all things, believes all things, hopes all things, endures all things.” (1 Cor 13:7)