The OP drives her mom to her appointments or when she needs to run tests
The problem is not paying for groceries, but being caught by surprise
OP has offered the following explanation for why they think they might be the AH:
I called my sister for spending my money without my consent. My mom said I shouldn't be angry because I also use the groceries.
The comments from other Redditors roll in...
They want the OP to pay a third of the groceries
The OP can help with the transportation
She should know better
The OP should take care of herself
The OP added this edit later on
I converted the currency very roughly because I don't live in the US and do not buy anything in dollars. But I spend around a third of a minimum wage in groceries every month. It's not a small amount of money.
The OP might be the one paying for the extras
The OP should move back to her apartment
The OP can't keep doing it all
They are using the OP
Taking care of your mother is a great example of being a loving daughter. OP's sister has to pick up some responsibilities and life skills because, despite the fact that OP is the one putting in the most work, her mother seems to support her sister.
Redditors understood that OP's mom is receiving cancer treatment and that she needs help, but the OP really needs to consider her own mental health. The OP was declared not the AH, and that's a wrap.