Legal Question in Real Estate Law in California

Unless your brothers are going to fight you on this, you don't even need to make a formal demand. The Section 843 notice is only a precondition to suing them for refusing to give you concurrent possession. You should try to work it out informally. If they refuse, then give the notice. No special form is required as long as you refer to the section. If they still refuse to allow concurrent possession after receiving the notice, then you probably need to hire an attorney. It gets complicated from there.

I agree. Negotiation after delivering them an 843 demand is the best path. If the negotiations do not succeed in relieving the ouster, then get an attorney. Your rights and remedies may not turn out to align exactly with your desires if you go to court, and a neogiated solution could be tailored to what you want. You are probably entitled to your share of the net rents received from the "third party" tenants, which is good, but you may not want concurrent possession of the entire property, which would make the brothers your involuntary roommates. As to attorney involvement, probably a good idea, especially if you do have to go to court, or to write up a voluntary settlement achieved through negotiation.