Your Father In-Law is still alive so technically your wife has not inherited anything yet, nor is she is the land owner, "unless" your wife has legally "segregated", portioned the land legally via a land title.
If your wife has not done this then the land on which the house you built is technically located on your Father In Laws land.
Is the land Titled, Tax Declaration or Salvage Zone?
You have no rights in this case nor does your wife regarding easement discussion with your future brother in law.
Your future brother in law is correct to deal directly with the Father in law regarding "easement' because the Father is still alive and is the land owner. Future Brother In-Law could demand a width enough to drive a vehicle to pass through especially if it's needed for livelihood, farming, etc. so tread lightly.
Suggest you read all of SECTION 3. - Easement of Right of Way Articles 649 onwards, BOOK II (FULL TEXT) : CIVIL CODE OF THE PHILIPPINES : CHAN ROBLES VIRTUAL LAW LIBRARY
Also, Google Search - Philippine Right of Way law and Philippine Easement Law and read articles such as this one.