I cant stand my girlfriends Roommate/friend?

basically my girlfriend lives in a hostel and has to share the kitchen and bathroom with a girl and theyve become quite close, i stay at my girlfriends alot so i gues you could say we all started to become quite close go out clubbing together all the time with a few others.. thing is i dont like her and only hack her for the sake of my girlfriend.... she can be extremely rude to my girlfriend sometimes and when people are there she will talk to her like an idiot.. she can also be quite bossy aswel about tidying up ever since she got her apprenticeship in a nursery she hardly makes an effort with my girlfriend and shes suppost to be her best friend.. when im not there my girlfriend will say shes on her own while her mate blanks her and has friends over in her room.. shes also become close to 2 other girls and ever since doesnt seem to bover with her much.. she seems to cook breakfast for them and completely leave us out despite me always offering her food.... she was borrowing money off her and not paying it back, asking my girlfriend to go out and buy her ciggarettes.. she can be quite intimidating aswel she will force us all to go clubbing with her ( i mean emotionally blackmail us ) as shes meeting 2 girls frm her work and then completely ditch us and go off dancing with them as soon as she sees them . ive also caught her giving us dirty looks a few times.. also she keeps bringing some unstable bloke into our home and treating him like crap but then pretendin to be his friend then bitching about how much she cant stand him yet she keeps letting him in, last time he was here he told her he hopes she gets raped and that he thinks his skitzo... yet she brings him into our home to argue with him and make us feel uncomfortable sorry for the long story.. ive told my girlfriend how i feel ... but i just dont no what to do, i honestly cant stand her.. shes embarresing to go out with shes an extremely over weight girl and she wears really tight underwear things wen where going out people will call her fat or pregnant which will cause further arguements.. also when shes drunk she spills drink and ash on the carpet and keeps threatenin to go to her ex boyfriends house to have a go at him... she is hell.. i dont get to spend much time with my girlfriend alone when im there cause of her