What does it mean when you dream of your living cousin being dead?

You are missing them. You may not be able to think about the person when you are awake, so your brain is taking advantage of this dreaming time to get your pain out and recognized. If you hold in the pain too long and don't deal with it properly, then minor and severe medical issues may occur, due to prolonged stress.

It depends on what exactly you dream in this dream. If you are hurting them in the dream, then you feel guilty. If you're looking on, and feel desperate to help but you just can't, then you feel hopeless. See a counselor if you feel very worried about this.