The laws in all state sin thia country is that those who have an ownership right to particular property no matter what percentage have the right to use it with all other owners unless there is a written agreement signed by the various owners to the contrary.In your situation, if your mother's spouse is on legal title to the property that you write about as well as you and your brother, all three of you have the legal right to occupy the property. None of you can be excluded by the others.As stated above, the only exception would be if there was some contract signed by two or more of the owners setting forth who would be allowed to occupy the jointly owned home.Good luck.