Compensation for Family Live- in Caregiver; suggestions?

My Sister who is divorced agreed to move in with my parents to assist them. When she first moved in they were able to take care of their personal needs and were able to be left on their own. She did the cooking and housekeeping. My Mom passed away 2 years ago needing more intense care the last 2 months of her life. My sister decided to sell her townhouse when she moved into our parent's large home. She currently is paid $2,000 per month and has all of her basic living expenses paid by Dad. She does not pay rent. All her food is paid for and Dad pays all utilities including cable and internet. She is given paid vacation time of one month per year, which she travels and is away from home. Dad pays for the additional caregivers needed while she is away. She also gets 1/2 of her gasoline costs paid by Dad. Her monthly pay is the total amount of Dad's monthly income. she does not work at an outside job. Our sister-in-law who is disabled and widowed lives there also and pays rent of $425.00. My sister complains of not being to go anywhere, but she goes to babysit and visit her grandkids several days a week, visits friends several times a week and goes to various church functions. Our sister-in- stays with dad during the day whenever she is away, but is not paid for her time. My other sister takes care of all the finances and pays the bills from Dad's account and takes Dad to most of his medical and Dental appointments but does not ask for any compensation. She is now demanding that Dad also pay her taxes and that she wants more money. I want Dad to be able to stay in his home as long as possible. He is not wealthy, but is still pretty healthy. My other sister and I want to conserve what we can so that his funds from his investments do not run out before he passes. There is probably enough to keep things going for another 2 to 3 years. dad is 91. My other sister and I have agreed to let her stay in Dad's house until everything is settled after he passes until she gets a new job and moves in with her son and his family, and to compensate her for the equity she would have gotten if she would have stayed in her townhouse. She helped her son get a house he could not afford on his own for which she provided the down payment and is cosigner on the mortgage, and is co-owner and is making half of the mortgage payment, which she really can't afford. She has no car payment, and her only real expenses are auto insurance and heath insurance. She also receives $400.00 from her ex-husband per month. Given all of this, I that what she is receiving is fair. She complains constantly about being underpaid and overworked. Dad sleeps until 10 am. She prepares meals for them and does the housework, which she would still have to do even when living alone. He keeps himself busy on his own, watching TV or napping. She verbally abuses our sister-in-law and my other sister if they don't do what she wants the way she says it. She also talked my Dad into cosigning in a mortgage loan for her son without telling us. He was so upset about it afterwords, worrying about what would happen as her son is irresponsible. Luckily it didn't go though, because of a mechanics lien on his house. We have tried to talk to her about all this, but she just yells about how much she does and deserves much more. Any thoughts and suggestions would be appreciated.