My mother and sister and ALWAYS having problems, and im always involved?

My mother is an absurd control freak. My sister enjoys going out with friends, and is very sociable. They always have fights about when she goes out etc. Yesterday it was one of my sisters friends' weddings, and my sister AND my mum were invited, which is very nice of the groom, as he doesnt even know my mum. My mum cancelled out on my sister last minute. My sister is really close friends with the groom, so she was going. Before she left my mum told her what time wil you be back, and she said the wedding finishes at 11. She was like well make sure u dont come home with a man?? My sister has NEVER done that. Ever. She always calls her a ***** and a prostitute etc. My sister didnt reply and went to the wedding. Her phone broke before she left, and shes not lying cos I saw it broken. She came on time at 10:30ish and the wedding venue was a very long distance away. All the time my sister was at the wedding my mum constantly questioned me about where she was what shes doing etc etc, even though she'd already told her. The next day (Today), my sister was going to a festival, and shes one of the main organisers, she needed to get her phone fixed aswell before she was going to leave, so she left at 12 at noon. My mum came and she asked her where she was going, my sister replied and she also said that she really wasnt bothered to go out, but she had to. When my sister got changed and was leaving, she said bye, and my mum shouted "YOU TOLD ME YOU WERENT GOING" which is not what my sister said, and she asked her what time she'd be back, and my sister replied with 9-10ish at night. As soon as my sister left, she started asking me questions again, with where is she going, what time will she be back, even though she'd just asked her?? Then she was like shes such a *****, and a liar, she says she doesnt WANT to go, so that I wont be able to tell her to stay at home. So i told my mum, why should she stay at home? Shouldn't she go out and have fun? And my mum had a real go at me, and threw stuff and broke glass etc etc. And she said my sister is satan, she'll "throw me down a well and leave me". which shows just how little my mum knows my sister, because a few weeks ago, i was broke and i didnt want to tell my mum cos she'd kill me, and my sister GAVE me �300, and not EVER mentioning she did so. She does so much for my mum, and I dont know why she's like this with her. My other sister who was sitting there just eggs my mum on, and said to me "didnt i tell u not to hang around with her?" but she's my sister, why cant i? From reading this you guys probably think my sister is like 16 or something. She's actually 28. She hardly has anymore friends, because my mum takes their numbers, and calls them to check if my sister is with them. My mum is mental. I feel so sorry for my sister, and usually im always involved in their problems. When im not involved, my mum will go right ahead and hit her. What should I do?