There is a vacant home across the street from me. It has been vacant for three years. The neighbor's brother wants to claim squatters rights and he has moved his personal belongings into the house. My room mate who is a Realtor researched the property records and the house is in probate. Can this guy be arrested?

Yes, he's trespassing. There is no such thing as squatter's rights.

If the house is in probate contact the executor and report it.