none as far as i can see shes legally an adult and therefore responsible for her actions. she knows whats hes like. she knows hes shaggin around - she will be treated like shit for a few years no doubt then leave him.
sorry - not good news.
the best thing for your parents to do IMO, is to make sure they dont put her into a situation where if things kicked off your sis would have no where to turn to - or run home to.
it would be awful if they kicked her out now, then he beats the shit out of her or something and she feels she has no one to turn to.
i would encourage your parents to encourage your sister to get her own place. help her search for a flat - is she working - can she afford it?
i mean as liberal as i am - i will not under any circumstances allow my kids to wonder in at 3 am if they are still living with me. becuase their pissed up antics would wind me up. i think at that age and for some time after they are oblivious to the fact that parents have a life too thanks you very much you selfish pissed up person.
so if finances allow - encourage her to get another flat. if finacnes dont allow encourage her to apply to council and housing associations for one bed or bedsit. at this point your parents can lie and say they are chucking her out.
then she will have to face the big wide world alone - and guess what council tax rent and any student loans are piority debt and theres no escape clause.
life will happen and she will wish she had her bedroom back
dont back her into a corner though or her pride may force her never to return.