my mother passed away and she has a will. but my sister took her car and has her son and his girlfriend living in my moms house is that legal?

Yes.  Take the Will and a death certificate to a probate attorney who practices in the county in which your mother lived and died and apply to submit the Will for probate (proving that it is the Will of the person who died).  Once the court appoints you executor, you can sue your sister for conversion (converting the property of your mother's estate to her own) and evict your brother and his girlfriend.