Hi,
I have an static IP at home.
I would like to use the PI-hole on my pi and be able to configure my parents router so it use my PI-hole on their network.
Of course their router is on another house. 
Which ports shall I forward to my PI to make sure it works?
Thanks
I recommend researching this site. Port Forwarding Your Router to Get Open Ports
DNS uses UDP/TCP Port 53.
I know how to forward the ports, I was not sure about the ports though.
Thanks!
You would forward port 53 (DNS) but I don't usually recommend this as anyone could portscan and find this and it opens up a world of security problems.
The preferred method is to use a VPN.