I would suggest that your brother-in-law see if his Mexican auto insurance has any extra coverage he can add to his vehicle for his 6 month visit to USA and California. Many Mexican car insurers appear to have what is called border insurance for trips into the US from Mexico.

If your brother in law cannot get the needed insurance that way you could next ask your car insurance company if there is anyway to add him to your policy. Without him having a California license it might not be possible but it is worth asking and finding out if your agent has any more information that could be helpful to either you or your relatives.

For California car insurance for yourself you can start the quoting process right now here with us.