Where do we start?

What does she think she'd be held criminally responsible for? There are many resources available, and help for your mom and sister. Have your sister call a local Area on Aging office, and go from there. She may even want to contact Legal Services (for free) for her mom, and if she or your dad was a Veteran, the VA is a good resource as well. That's a partial list, but a good place to start. Best wishes to you all. Nice of you to be looking out for your mom and sister.

jbunker.. take a deep breath! to start.. has your mother been deemed unable to live by herself? what does her doctor say about her general health? Next, have you talked to adult protective services(adult social services)? Calling them isn't turning yourselves in for anything criminal! They have resources that I personally had never of.
Here is the link for Florida http://www.dcf.state.fl.us/as/ "The Program also supports adults with disabilities who need assistance to remain in their homes and/or in the community." Give this a try! I did and it was the best decision I ever made when I lived out of state from my mother!