My little sister wants to date my boyfriends little brother?

My Joey Bear(bf) and I have been together for about a year!!(sleep together,...whole shabang) well resently his brother(16) my little sister(15) have been hanging out and talking alot more than normal...i've seen them kiss and be very hormonal teenagers........i'm starting to think they wanna do "the deed"() because they really like each other.........NOW I CAN'T HAVE THIS GOING ON!!! I'm 19 and so is my boyfriend........he proposed to me a few weeks back AND I CAN'T HAVE MY LITTLE SISTER AND HIS LITTLE BROTHER ******* AROUND WHEN WE'RE GETTING MARRIED! what do i do to stop them? it would be better if they WEREN'T gonna be cousins but how do i stop them? (i know they're dating for sure, but don't know how long this catastrophe has been going on)