marraige of HIV persons

Hello sir/madam, my name is Divya. I"m here to seek your help. I am having a sister who expired of HIV. She is having a son. Even my sisters husband has HIV. Now the son is living with his father. father having HIV is going to marry another women having HIV. his father has written an agreement "even if he(father) expires, the second wife is going to stay along with his son and her own daughter in one portion of her house..and continue her surviving with rent of other portions of same house. So my problem is that..we cant guarantee that second wife gives money and make her husband son to i just need a legal advice so that i want to give a good future to my sisters son. And let me know whether legally can i prevent marriage between my sisters husband and second wife...please i have only five days to fight against him..because his marriage is going to be held in 6 please suggest me as early as possible. please send your suggestion to my ID is thanking you