At 5, they need help from others in both standing up for themselves and being taught how to interact with others.
Be present when they are together, not hovering but close enough to intercede on your son's behalf. When C takes something from him (and your son doesn't say anything) you can say "C, that's not nice. You need to give that back to Joey and ASK him if you can play with it when he's done". You may need to step in to actually facilitate that if he isn't cooperative (but most kids are when it's an adult/other parent). If he hits or is aggressive, you can say "C, that is not acceptable. We don't hit"...if it happens again, you walk him to a seat or another area and tell him that he can resume play when he can keep his hands to himself. Be CALM and just matter of a fact.
While it is his mother's job to correct his behavior, it is your job to protect your son. Additionally, sometimes it takes a village to raise our children. Since you are close, I can't see where any of this would be a problem...wouldn't you want your cousin to help with teaching your son if the roles were reversed?
Not only will you be teaching your son you will stand up for him, it models how he can stand up for himself, it teaches both boys acceptable behavior, AND models corrective approaches for your cousin too.