LAND OWNERSHIP issue

hi i just want to ask if we(our) family has a right on our grandfather's land here in antipolo..

first, my grandfather has a land here in antipolo he got a kerida or mistress. they are not engage but the name of the land was transfered to her.. in her name..

now my question sir is this
1.do we have right to claim the land that is under her name?
2.can we file a complaint because we already live in the house which was built by my grandfather for 16 yrs now..
3. if we can, where do we file it?
4 do we have to hire a lawyer and how much will it cost to get the help of lawyers?
3. she doesnt; want to negotiate with us what can we do?


here is our situation.
1. my father is the son of my grandfather
2. my grandmother is still alive
2. my grandfather already pass away
3. we still live in the house for 16 yrs now and the land occupying our house is under his mistress name..

thanks and Godbless