Second marriage

hello frnds. my problem is regarding my cousin brother in law.he is madly love with me.bt at the same time he loves his wife tht is my sister too.and we are having physical relation too.he is very serious about me..he wants to mmarry me and will keep two wives at the same time.my sister also agree with him. i was earlier too very serious.my age is 25 and his 38..but now i cant do this.i cant left my parents .and the society also not accept us.but he is like dying for me.always keep telling cant live without yo.he wants tht i shudnt marry for whole life.plz tell me .how cud i settle this thing.he is disturbing my life too.m a doctor b tgetting mental.