If your Father In-law's land was "titled" and subsequently legally partitioned and surveyed, the survey drawings "should" indicate the easements (right of way) for each of the 3 portioned lots.
The Boundary Agreement Process (BAP) and the Parcel Information Sheet (PIS)
b. Sketching Instructions
3. All easements must be shown, including dimensions;
I'm assuming the land is other than titled and was divided 3 ways. (This rock to that tree).
Title III. - CO-OWNERSHIP
Articles. 484 and onwards.
BOOK II (FULL TEXT) : CIVIL CODE OF THE PHILIPPINES : CHAN ROBLES VIRTUAL LAW LIBRARY
Your easement issue with your future brother in-law is likely the least of your issues you may face.
I'm aware of situations where 1 co-owner pawned their portion, lost it and it the new owners created chaos as a new co-owner.
To many "what if's" in this scenario. Particularly "what if" the 3rd sister marries a local, he uses their 3rd of co-ownership as collateral for a loan for a trike, gambling debt, etc. then loses it.