M.N.
Definitely sounds like you have your hands full. There should be services to help with the majority of the issues though you just need to find them.
For all of the adults (Step father and both mentally handicapped sisters) medicaid should have some type of services to help you. If Medicaid doesn't have anything then possibly contact Adult Protective Services, they may be able to help you also. If you are not comfortable contacting them them for information then possibly (I don't know if it would be beneficial or not) contact a nursing home and just ask them if they can send you in the right direction to get some help with services. Also, if your step father ever served in the military then he should qualify for vetran's services.
As for your client, I understand how hard that can be. However, you probably do need to contact Adult Protective Services regarding her situation. They will investigate and will not let her starve. Yes she may end up in a home but she will not starve.
As far as your niece, contact CPS again and/or contact a lawyer, if nothing else her father should be paying you support to care for his daughter. what about your nieces Mother? Why is she not in the picture. You absolutely have to do something with the situation with your neice legally though because as of right now you have not authority to get her any type of medical help and at the bare minimum you need that.
It sounds harsh but honestly your main #1 concern needs to be your 3 children. They need to be your main focus. I understand that everyone else (almost) involved is your family, but your children need to be your #1!
I do feel for the situation you are in. I hope that you are successfull in finding some services that can help everyone involved.
As far as the rude comment - don't even give her the time of day. Maybe she is just having a bad day and decided to be snarky and nasty and take it out on you. Not worth your time or your response!
Good luck and keep your head up.