What do I do when my step-sister is giving my mom Valium and now my mom thinks I’m bad?

I'm 50 yrs old (only child of mother and father together) have step sister and brother. I live on one side of mother sister lives on other side .I have to work , sister does not (9 yrs older than me)therefor she takes mom to dr more than I..except when mom gets really ill then it me who takes mom to hosp stays there with her(sister cant says she has grandkids to take care of) I have grandchildren also.Lets see if I can make this reasonably short,Mother was hooked on lomotil,sister was trying to get more lomotil for mom, mother was having terrible bowel probs, Some days ,my days off work I would have to give mom as many as 3 inemas to clean her out, She would lay on the couch in agony until then. when I was off work and able to take mom to dr myself, mother asked for more lomotil refills, my sister sent it with her to give to dr, when I told dr what had taken place he said he would no longer give her the pills it was paralyising my moms bowels, 4 days later my sister had mom convinced I was wrong in doing so (mom mad at me now) then we made it through that after mom calling me and telling me how bad a person I was and how I did'nt care if she was in pain (for lack of lomotil) , She gets sick again in hosp again (same reason as before Pouring blood out her rectum so much as last time they had to give her blood.) anyway I stay w/her in hosp , her state of confusion before going to hosp was apparent) she sees pshciatrist who gives her vallium. reg dr knows this and does not agree with him, anyway we weaned mom off of vallium while in the hosp and my mother finally became the mother I knew. for 4 weeks I fixed breakfas lunch and dinners for mom (she had special diet) I I made them up in sacks so when it came time for mom to eat all my sis had to do was give one to mom, for 4 weeks my mom was doing great, Then the new wore off( or something?) my sis takes mom to dr for checkup, Came home started fixing mom wrong things , greasy ect ect. was still staying with mom nights I saw my mom was reaching for a pill laying on her nightstand I asked my mom what she was doing she said my sis layed them out for her to take ,I was giving my mom 2 tab of vallium at night to help her sleep only then. well anyway my sis was giving her them duiring the day just in case she needed them (of course mom said she needed them if offered to her) so anyway since she took 1 at 8:30 that my sis had layed out, I took one of hers away at night since it was so close to bedtime. My mother got mad said things to me ect, By now seeing my moms state of mind I kew it was too late , she had been taking more than I knew of thanks to my sis, I had to come home that night I was so frustrated that I said mom I can't stay here and watch u destroy yourself, I came home and called my sis and told her what happened and in a mild voice said "I just don't understand why you have give mom these pills , you know that they make mom not think straight" (left on ans. machine) a few minutes later she called back and left a nasty message on mine. I had told here that if she didnt want mom to stay the night alone she should go there because I was to upset, ( one thing on her nasty message was "hey if you don't want to stay with mom just say so u don't have to make excuses") this is after being w/mom everynight and on my days off and trying to work (It was hard but I love my mother and she needed me, she could'nt even get up to go to bathroom or wipe and I gladly did it. anyway a few days had gone by 3 maybe and I couldnot take it any more I had to call and check on mom. sis was there (almost sticking like glue at this point, except nights) she was yelling at me in the background I had to tell mom goodbye and I loveher and would talk to later call me if she needed me. 2 more times I called and the same thing happened, I told mom to call me when she could talk to me, but once again I knew it was 2 late mom was accusing me about money situations and saying my dad did'nt leave her any money when he died==not true she has thousands of dollars in bank (in my sis son name and moms) but this is not about money its about my mom. went to visit mom today sis was there what happened -you can imagine- I tried to stay calm for my moms sake and just let my sis go off saying how I did not care about mom.and let me know how many days it was since I went to see mom.( this is my church go'in sis might I add) anyway mom is very sick again begging me to get her some meds (that I know would make her worse) and my sis has her convinced (which I do'nt blame my mom, she's confused at this point)that I dont care about her, and my mom said she don't know how I can see her in pain and not give her meds. Said she would,nt treat a dog the way I treat her, yes my mom has drug prob but my sister is keeping her drugged up. my mom now is not even aware she has money in bank, she needs glasses but says she can't affor them ---How do I handle this , what am I to do ? I love my Mother.