Regarding sass: we have a "penalty board". Each child has a hook. We have lots of penalty cards that have chores on them. Obnoxious arguing gets them a penalty card. They have to clear their hook before they get any screen time or sweets. Sometimes it takes them a week. Usually the little ones need help doing their chore. We spend the whole chore (like cleaning the toilet) talking about *why* they have the chore.
Regarding rough play, is someone actually getting hurt?
If someone is getting hurt, define it as a problem and ask your older child to think of solutions. ("Wow, baby brother is crying. Do you know why? How do you think we could make sure he doesn't get hurt again?) Also, involve your older child in caring for his baby brother when he gets hurt. He could bring an ice pack, offer a hug, give a stuffed animal, etc.
However, if everyone is having fun and you see no immediate danger of serious injuries, let it go.
It was my 6 year old that discovered that her baby brother loved having stuffed animals thrown in his face. It never would have occurred to me to chuck a teddybear at my infant's head, but he loved it. It became one of the ways she took care of him for me. I'd make dinner. She'd sit there and throw stuffed animals at his face. He'd cackle with delight.
Kids are sometimes very strange.