Hi Bebe,
I have seen this play out in so many families, where one child takes care of everything while the others just show up only now and then or sometimes only in the end when the will is being read!
It is a difficult position to be in when siblings do not share the responsibility equally. It's their mother too. You're doing a selfless act taking care of her singlehandedly. It would be hard not to feel resentment. Don't be too hard on yourself.