I would like to block some websites on the guest account, so my cousin cannot browse things that are not safe for my computer.
You can do this using Parental Controls on the Guest account. See Apple's KB article regarding this for the procedure:
But to fully protect the computer from potential harm from Internet regardless of account used one has to block any downloads and installations without explicit approval. (I do that to protect my self from my self :)
Each browser has they own set of tools that can be used to protect user and computer. Loots of good add-on's in Firefox.
As an example for Firefox you would use a add-on ..Block downloads,
When you use the Guest Account feature, it is isolated from the rest of the system. The guest user has no administrative rights and anything that is saved by the user is deleted after logging out. You can fine tune what the guest user has access to (this is a good article) but AFAIK, and in regard to whether or not your cousin can download something that would be harmful to your computer, the Guest Account is completely safe to use.
According to Apple:
and with even more restriction:
Unless your cousin works for the NSA (I say this tongue-in-cheek), I don't think you have anything to worry about if he is using the Guest Account feature.