I have been using a sim for two months on my iPhone and now I gave that sim to my cousin. What I am worried about is will she recieve my WhatsApp chat history when she downloads WhatsApp to her phone.
WhatsApp works on the phone number, not the phone, so you need to transfer your number
From the WhatsApp FAQ
Before you stop using a particular phone number or phone, you should migrate your WhatsApp account to the new number or new phone.
For a simple way to delete your old account number and migrate your data to a new number, use our Change Number feature.