Family members taking advantage of parents medication

My parents are both old and in very bad health. My mother has crippling arthritus and my father has a back disease, they are in their early seventies. My sister has been stealing their pain medicine for awhile. They get like one hundred each for a month. And they are out with in two weeks,sometimes sooner. There was even a time my sister took her daughters prescribtion to get it feeled for her and was supposed to take it to her the next day. (Her daughter lives approxametly 30 miles from her. Because she is liveing with my oldest sister. ) When her daughter called her the next day to tell her her leg injury was hurting severly, she told her she got her prescription filled and someone stole it from her car.This has been an ongoing thing for about four years. She and her husband now left a well paying job out of town and moved back in to my parents home. They now live there with my parents. And my dad counts his meds before he goes to sleep and wakes up to thirty or fourty sometimes all the medicine is gone. There has been instances where tylenol has been put in place of the medicine. My parents know who is doing it. And they know there is a major problem but they won't do or say anything about it. And if i try to do anything, they get mad at me, and even go as far as saying maybe i am the one taking them. They have caught her on several occasions, and i am not even there overnight when they come missing. It has caused so many problems that i have lost respect for my parents and i know thats wrong but i am very emotionally hurt through it all. I dont know where to turn anymore. I am trying to save money to take my kids and move away. Which will cause us to loose the home we are buying. But, i just want to find happiness and get away from all the evil lyes, stealing and head games being played. I fill it is not setting a example of the way families should deal with things. And it has tore our family completely apart. I would love some advice.