You will need to surrender your registration if the plates have been stolen.
No someone might have taken them but I'm going to go look for them later.
If I do determine that they have been stolen, how do I get new plates?
You will need to report that the plates have been stolen and surrender their registration. The DMV can then issue a new registration and plates.
If I find the plates can my brother-in-law bring them in to prove that I got them back?
Yes but only if he intends to surrender the plates for you.
Maybe I don't trust him so much. Can my little brother some in instead?
Yes as long as he will surrender the plates for you.
He's probably too young to understand forms and stuff. Can I fill in some of the paperwork online?
Yes there is the Complete the PD-7 application that is available as a PDF online.
Can I submit that online or do I have to print it up and give it to him to bring in?