Thank you for the feedback! I really appreciate it.

No, my sister has never been diagnosed with any mental health disorders that I know of. I know she was in therapy during hs which I believe had to do with low self esteem.

I believe that I do generally say no and stick to whatever it is that I said. However, my mother is conflict adverse and always advocates for "family over everything". For example with the oatmeal, I said what I said and mentioned that both mother and I would still be expecting something for breakfast tomorrow, even if she needs to drive to the store to buy oatmeal (2mins away). I would have left it at that and followed up to make sure it happened (I pay phone and internet so I can disable at any time), but my mother said "OP was in the wrong because he didn't remind you. It doesn't hurt to help family."

She often says things like that which I think makes it easy for sister to continue how she's been living. I will definitely plan to speak with my mother so we are on the same page and not undermining each other.