If either of the parties requests a venue change to Los Angeles, it will probably be granted. If he wants orders sooner rather than later he should file first and worry about transfer later. It can take a long time for a file to make its way to another county. If she files for support modification she can file in Riverside or if she uses the DA or private counsel choos, the judgment can be registered in LA and procedings take place here.
Brother needs to deal with the issue.
The longer he drags this the deeper the debt that he one day will hope to climb out of. These problems do not simply go away. If anything I expect that enforcement will continue to be a high priority for the gov't. The issues are easier to deal with the earlier he takes an active role. Santa Clarita is the County of Los Angeles.