You are absolutely correct in your desire to finally get married. Your boyfriend is also correct in feeling responsibility for his ageing and demanding mother. BUT life must go on because that is the naural run of things. Your bf must meet with his sister and tell her that he has done as much as he can and she should either take care of her mother or agree to move her to a nursing home. Otherwise your lives together will have no meaning and no purpose. Keeping his mother in his home is no longer acceptable. If your bf refuses then you should leave and look for a better relationship; this way will lead to a dead end for you and him.