Legal Question in Real Estate Law in California

Diplomatically tell him that they are your things and you need to get them back. You are willing to work out a compromise in how do do so, but if he will not return them you have no choice but to sue. On a specific day he can move the items to the sidewalk area and you will pay for an lindependent mover to pick the items up and bring them to you [or if he is willing you will be there when he starts to move the items and you will put them into your vehicle].

Unfortunately, in Small Claims Court you can not sue for return of any items, only for their cash value.

If friendly persuasion doesn't work, there is a type of lawsuit that fits the situation. In the old days it was called "replevin" but California's Code of Civil Procedure now refers to it as a lawsuit for "claim and delivery." Basically, it is an action to recover possession of personal property wrongfully in the hands of another. Perhaps the brother-in-law would give you access to your things after receiving a letter from an attorney indicating that a suit would be filed in, say, 30 days unless arrangements were made, and carried out, for return of the property.