My wife has teenage daughters from a previous marriage. Until recently both children lived with us as my wife has joint-custody, primary residence (I believe that is how it's defined) nonetheless, the oldest moved in with her father & they (bio mom & dad) agreed to reduce the CS by half. Now the other child wants to move in with their dad. Again, if this happens, then I'm assuming they would again agree to him not paying her (my wife) any CS. Our fear is now the father will petition the court to order us to pay child support to him since essentially the roles would be flipped flopped. My wife has no income, stay at home mom for our child we share, therefore I'm the only one producing income for the family.

My question is: Does he have a winnable argument? He has never been an absent parent, he sees the children on his normal visitation schedule plus extra visits when schedules permit. Yes, I've been a "parent" to the children, but purely as a step-parent. We live in CA for legal reference.

Yes he can collect child support from her. Why shouldn't he? Also they may base it on the last job she has held.

Child support is paid by the non custodial parent to the custodial parent for the upkeep of the child. So, if he was paying support while she was the custodial parent, then why shouldn't she be required to pay support now that he is the custodial parent.

The fact that she is unemployed will limit the amount of support she is required to pay.