Claim of an illegitimate child to property owned long before he was born

Good afternoon FLAP. I am the eldest of 4 children. My mother died in 1986 while my father died 2 years ago. They were legally married sometime in 1964. Before my mother died, they were able to acquire properties which included the house where 3 of us are living right now and several properties in Bicol not to mention 3 fishing boats. Three years after my mothers death, my father had a relationship with a widowed neighbor. They never got married but they had a son who is now 18 years old. However, during this relationship, beyond our knowlege, my father sold all our properties in Bicol. This woman died 4 years ago. This illegitimate son is now claiming support from me and my brothers besides his intent that the last property( our house) should be sold so that he can get his supposedly 'share' of my parents property. This seems unfair to me and my brothers considering that we never got any share from the Bicol properties that should have been equally divided among us. My brothers and I never intended to sell this house since this is the last remaining property my parents had left us. The house & lot is under my mother's name. Does he have the right to this last property owned by my late mother considering it was her mother who duped my father to sell all my parent's properties in Bicol beyond our knowledge? If you analyze closely, this illegitimate son got more than us legitimate sons. Please advise what legal action we might face considering he is already 18 years of age. Thank you...

IMO, the said illegit child has no right on the property left by your mother but he has in terms of support (no specific amount and depends on your capacity) from you as his siblings.