Jointly own house w\Brother and one w\ Mother, primary residence?

[Kjeld Pedersen]

Had a lot of changes in my life, and used most of my savings to get a place for my mother recently, previously I've bought a place with my Brother 12 years ago where I lived full time, and recently got married as well. Both are mortgages located in the bay area, California. Currently deciding where to live.

I would like to know for financial and tax reasons what would be my best option, I was looking and my mom as a senior can qualify for benefits as a primary resident to her new home, all bills go under her name at the place, although haven't broken down numbers to see if it matters, the interest on the place owned with brother is 2% higher, their debt ratio are roughly the same.

Where would be best to declare primary residence and choose as home, or do I even have that option, as my mortgage already dictates where is primary? Second does what is declared on the mortgage is the same as what is declared on utilities and other bills? Is there a difference between primary residence and where you actually live or they are or should be the same?

Thanks in advance!