Someone has moved into a mobile home my sister in law has the title to.?

[lil_julbug]

It's a bit complicated. My brother & sister in law have a clear title to & have lived in a mobile home for at least two years that is on property my grandmother owns (but does not currently live there). My mother claims to have control of it. My brother & mother had a falling out and are currently in the process of moving. Mom left a note on their door saying they needed to get their stuff. One day later my sister moves in even though their things are there. As I said they have the title to the trailer. Is his sister tresspassing? Do they have any recourse?