"My spouse cheated on me, and now this person is living with my spouse and our children. Is there anything I can do about this - and will this affect any of the issues in our divorce?"
Some states still allow "fault" to be a factor for determining some or all issues such as spousal support, property division, and attorney's fees. California, however, is a "no fault" state. This generally means that the fact that your spouse cheated on you can't be used against him or her, in any fashion, on any issue.
Likewise, the fact that this person is now living with your spouse and children has very little effect under California law. Conversely, some states will often grant restraining orders preventing a spouse from having a boyfriend or girlfriend overnight when the children are present. California does not allow such orders under most circumstances. In addition, if your spouse is financially supporting this new person or vice-versa, California courts do not consider the financial impact of the new person on the issue of support, unless you can demonstrate that your spouse is suppressing his or her earnings as a result of this new person.
Good, bad, or indifferent, California made a decision years ago to remove (as much as possible) "fault" in divorce proceedings and to simplify the analysis of support by generally not considering the impact of the new person. The only exception is, if it can be demonstrated that there is something seriously wrong with the new person (e.g., mental instability) and that this has a negative impact upon the children, then the court may take action. But generally, there is nothing to be achieved in raising issues about the new person.
Mark P. Gross is a divorce attorney and a founding partner in the Sherman Oaks-based firm of Brot & Gross. He appears in all of the family-law branch courts of Los Angeles County and Ventura County.