Problem with iTunes Store and updating WhatsApp
It sounds very much like the phone wasn't wiped &amp; set up as your own new device.
This means that you will need the old login data for everything you need to do on the phone - this is not good. 

You will need to get your cousin to make sure the phone is removed from his iCloud account, then you need to wipe it &amp; start afresh.
You will lose all existing data, but currently, nothing on the phone belongs to you.

See What to do before selling or giving away your iPhone, iPad, or iPod touch