Legal Question in Real Estate Law in California

You have way more of an issue than can be addressed in a free internet Q&A forum. You've started out right by contacting your insurance company. You should also notify your neighbors that they need to contact their insurance companies because yours will be looking to theirs for reimbursement. What you do from there depends on how responsible, responsive and cooperative your neighbors are. It is a serious act of trespass to discharge storm water onto a neighboring property other than what naturally flows in a channel created by nature. You have the odd twist of the fact that the improper discharge of the water is originating on property that is not contiguous with yours. What that does legally is makes the water from the first neighbor into the water of the second neighbor and makes the second neighbor liable for the trespass because it crosses his property with his consent. Hopefully you won't need to get into all that. What the neighbors need to do is come up with a plan to collect the water from the pipe before it reaches your property and discharge it elsewhere that it is legal to do so. If the neighbors are sensible they'll make this a civil engineering and landscape architecture exercise, rather than a legal one, your insurance company will cover your damages, and that will be the end of your problem.