First is you must realised that she cannot marry you till you convert to her religion or if she would convert to yours (which i dont think she will). So you must sort that issue first and foremost. Secondly, her sister can be allowed only for a visiting visa or a student visa...but that would cost you money. She cannot have any other visa than that as she cannot be petitoned due to her age (21 on 2012), if it is a visitors visa it will only allow her a couple of months to visit you but she will not be allowed to work. Student visa, yes she can work but only 20hrs in a week but remember she must enrolled in a college. and lastly is she will marry another brit