If he really wishes to marry you then he should tell this to his parents. He has to tell them that he is going to marry you by the end of this year, by that time they should get her sister married. If they donâ€™t, then they have to bear the consequences. This will put pressure on his father to get her daughter married. Or otherwise you have to start withdrawing yourself, this may put indirect pressure on him to become active rather than just passively waiting for his sisterâ€™s marriage.