Do I have to pay child support?

It is a little difficult for me to follow your question, i.e. which mother is causing the problems.  In any case, you will not be liable for your husband's child support obligations.  The only way it might impact you is in relation to his tax filing status.  California uses a statutory child support guideline to determine how much child support parents must pay.  This formula includes both income and tax deductions.  So, if his taxes go up because of your marriage and resulting change in tax filing status then his support might go down.  If his taxes go down, his support might go up.  The amount he must pay is based in large part on how much net income is left to him each month. You can further protect yourself by having your income deposited into a separate account so that in the event that she seeks to collect a child support judgment against your husband's accounts your money will be protected.   If she files with the department of child support services your husband should be prepared to turn over documentation reflecting the amounts that he has paid her voluntarily to ensure that any arrears are correctly calculated. Finally, child support isn't taxable income, so your husband can't claim it on his taxes as a deduction and she isn't required to claim it as income either.  However, he can make the case that her free rent should be attributed to her as income, but I would suggest that he consult in person with an attorney to discuss these more complicated aspects of his case.  I hope this helps you! Best of luck.