She is a fall risk and uses a walker and has a 'Lifeline'. I do all of the shopping, bathing, bill paying (with my money) and I'm there whenever I am not at work - I work full time.
My sister stops by once a week to take my mom to get her hair done and drives her to her doctors appointments, only because she schedules them, doesn't tell anyone when they are until the day before or of, and makes them at a time that is not reasonably convenient, due to my work schedule, for me to take her.
Who is her primary caregiver?
If there are money issues being argued about (and I suspect this is it), the two of you need to calmly sit down and discuss. Who manages your mom's SS money? It should be going into a separate checking account and not mixed with other money. BOTH of you need to know the amount of $$ your mom has and also both need to be on the contact list for all her doctors and medical info.
On the face of it, you are your mother's daily support and mainstay and you would seem to be the primary caregiver. But it isn't that simple: partly because your sister handles medical matters, and partly because even say one of you takes precedence over the other, that still doesn't give either of you the right to take control of decisions that belong properly to your mother or to her formally appointed agent (e.g. POA or health care proxy).
Do you mind if I ask what's going on? Maybe we can help.