Scrubby, you've got a lot to deal with so I'll try to put this delicately: it sounds like your sister's advice-giving is only the tip of the proverbial iceberg. She sounds like an animal hoarder, which, if memory serves, has recently been listed as a mental disorder in the DSM-IV manual.
It's not normal to have 20 cats and/or dogs, especially when you can't take care of them properly, and in many states and counties it's illegal. Where I live, it is not only a considered a mental health issue, it's a criminal offense for which you can be fined, sent to jail, and/or forbidden to have pets at all for a prescribed period of time, usually five years. I'm really sorry about your mom's kitty; my own family seems to think RN=DVM and they always want me to fix up our pets when something's wrong, but I know my limitations.......I am NOT a vet!
Your sister needs some help. I hope she gets it, and that her poor animals can be cared for properly. You're right, she's hardly in any shape to be giving advice, but that could be just a symptom of deeper issues.