Why does my boyfriends brother do this all the time?

Ok so my boyfriend and I have been together for 7 years he has a little brother that will be 5 in July. My boyfriend and I have a 6 year old son. alot of times that my boyfriends brother and my son play. his brother does nasty stuff to my son. I always walk in on them getting ready to do something. Just now I walked in on my son bent over and my boyfriends brother behind him. I asked him what the hell they were doing and my son said that my boyfriends brother was gonna lick his but. wtf is going on. This is sickning. I told my son he has one chance to tell me the truth or he losses everthing. he said it was the other kids idea. and he does not know why he does it. please can someone tell me what is going on. I can keep them seperated but they live next door to each other and they are family. please I dnt understand. OOO btw my son is not just the victim. I have caught him a loonngggg time ago trying stuff with one of my friends kid. but that ended asap!