Legal Question in Real Estate Law in California

Well, technically, yes, through a legal procedure known as a lawsuit for partition.

Partition lawsuits are the basic legal mechanism for unwinding unhappy co-ownership of real estate (or, for that matter, personal property as well). In a partition suit, the plaintiff asks the court to order the sale of the co-owned property and then, upon sale, to distribute the net proceeds of sale (after expenses and payoff of loans and liens) fairly to the former co-owners.

The action is referred to as a partition because, in earlier times, the court ordered a physical lot-split and each former co-owner ended up owning a fraction of the property. Nowadays, such a subdivision is generally impossible because (a) there is just one house on a small city lot, and/or (b) the modern laws regarding subdivision of parcels make it unlawful to further subdivide the parcel.

This is all background information, but possibly useful background. Other than a possible lawsuit for partition, the 5/6 owner (a) can sell his 5/6 interest to Joe Shmoe without your permission or knowledge , but (b) cannot do anything to upset, un-do or otherwise compromise YOUR 1/6 ownership. This assumes that the two of you have not entered into any kind of contract altering your rights. I also assume there are no other unusual facts that you didn't mention in your one-sentence question.

If the property is not legally physically divisible the court can order it sold to a third party and have the proceeds split based on the amount of your shares.

Yes, any owner can file a 'Partition' action for sale and accounting of the proceeds. You'd get your share.

Yes in a partition action.