A.C.
When my step-dad's ex psycho was trying to get 1/3 of his income (because she COULD ... not because she needed it) she also tried to include my mom's income in that. And the courts said that my mom's income wasn't counted because they weren't HER kids so not her responsibility. This was probably 15 years ago or more and in Ohio/Indiana, so things may have changed since then.
Generally child support is based on the person paying it and their income as well as state minimums, needs of the child(ren), etc. And changes don't generally get made unless one party requests an evaluation or revision of the order.
But this IS california and the rules that apply everywhere else ... DON'T here LOL.