If I am trying to have someone removed from a property I own and there's no lease and no rent is paid, do I have to file an eviction or can I just change the locks?

If you want to help your brother in evicting solely your sister-in-law where no rent is paid and there is no written lease with her, you cannot legally change the locks to the unit. To do so would expose you to great liability.

The way to remove your sister-in-law legally is to serve her with a 30 day notice to vacate. If she does not vacate within this time period, you then file an unlawful detainer action (eviction matter) and serve the summons and complaint upon her. If matters require the filing of an eviction action, I suggest that you consult with a real estate attorney to assist you.