Legal Question in Real Estate Law in California

Property taxes are assessed to the owners of record, based on an appraised value derived from the reported purchase price and a combination of other factors, to which the assessor applies a percentage determined from Prop. 13, then adds in other items such as per-parcel taxes and special assessments. If you first appeared in the records of ownership in 2008 by "adding your names to the title," that's presumably when the county began billing you for the property taxes, again presumably based on 1/2 of the 2008 value of the property. When you became the sole record owners in 2009, the billing would approximately double, since you'd then be billed for the other half interest at its 2009 value. The county doesn't care about your paying the taxes between 2003 and 2007; they weren't billing you for them then (I suppose), and as far as the taxing authorities are concerned, the payments were coming from the sister in law and her husband, since as holders of record title, they were the ones who were assessed and obliged to pay. (This is all assuming I understand the facts correctly).

To clarify and simplify Mr. Whipple's correct answer: under the origial Prop. 13, property taxes are reassessed whenever ownership changes on the county's title records. It makes no difference that you had some rights in the property, whether legal or equitable, before that. It was the act of taking title that gave the county the right to reassess you. So you are very unlikely to win your appeal. There was nothing improper in the reassessment.