Statute of Limitations on pursuing a property that is yours and squatted on is,
I believe, 20 years. If your mother has proof that she commenced legal proceedings
to have her property vacated by others within the past 20 years, she has a chance.
Also, if your mother is within those 20 years, then she will go to court to prove inheritance AND maintenance of said property for the past 50 years (1950 to now). In many
cases the squatter may win by proving maintenance and upkeep of property.
Otherwise....kala ksipnitouria!

FYI: I am not an attorney in Greece.