Since the vehicle is in his name, car insurance companies won’t see you as having an insurable interest–you don’t have a personal stake in what happens to his car. Since insurance is risk-based, insurance companies need to know you have a reason to insure a vehicle, like ownership.
If your insurance company doesn’t allow you to insure his vehicle, you will likely still have to list each other as drivers or household members on your policies. Even with separate policies, insurance companies want to know who has access to a vehicle.
If you want to be on a policy together, shop around. You may be able to find a company that will allow you to insure your brother-in-law’s vehicle as well.