Help!

Hey guys I have 5 kids Joshua and Lucas who are 2 and a half , Riley who is one year and 3 quarters , Rory who's one and kayden who's 4 months. My oldest two boys have always loved being big brothers until around a month ago when they started bullying there brother Riley around a week ago it stopped but on friday night I was cooking tea and then I walked into the living room to see josh holding a pillow over his head and then Lucas kicking him! I have never seen them behaving like this I'm starting to  see the same behavior towards kayden and Rory and I'm really worried any advice..