Hi Brokensemaphore,
Adding drivers to a car policy is completely LEGAL. You have nothing to worry about there.
Looking at the rest of your post - you needn't do anything really. By that I mean, once your father in law says all is done, ask for the certificate of insurance - so you can make a copy for your records and also, that you want to make a few copies including one to keep in the car in case you ever get stopped by the police. This way, they'll not ask for you to produce your license and insurance details at a police station.
If he umm's and ahh's at that point, you know something is up - if he's done everything correctly, including getting the policy documents which show the figure for the premium WITH your two names added, he'll have no issues in showing you the certificate. You really dont need to waste your time ringing the insurance companies asking and double checking stuff.
so, get father in law to find the best price quote for insurance and tell you the figure before accepting it to make sure you can afford to pay the increased amount. Once you decide to press ahead, just ask for the insurance certificate once it comes through. Make copies and keep one in the car to show any coppers you are insured.
Hope this helps!
DtM! West London & Slough UK!