Marriage against brother

Hi Sir, I really need help... myself sumit kumar(age-28) and my girlfriend(she is my brother sister in law) age is 25, we are in relationship from past 9 years, i have 2 brothers , my elder brother whose sister in law don't have any problem to marry but my another elder brother is creating issue to not to do marry with that girl and he is threatning lots of times that if i'm going to do marry he will do some criminal act ...every one is ready in my family except my one brother ....so please guide me what to do?? and please guide me the act regarding this matter . i am egarly waiting for reply thanks in advance