Same answer as before.
https://ttlc.intuit.com/questions/4149926-how-do-i-establish-the-cost-basis-of-a-house-i-built-30-yr...
If you can't find records, you can make an educated guess, and hope you aren't audited.
A real estate agent or home builder may be able to give you a ballpark figure on how much it would have cost 30 years ago to build that style of home.
However:
You also need to check the deed. If your wife and sister inherited the house, then the cost basis is not the cost to build, but the fair market value when their father died (which a real estate appraiser can help with.)
If your father gave them the home while he was alive, and the deed was "in fee simple", then the house was a gift. Their cost basis is the same as his cost basis -- i.e. what he paid to build it, plus the cost of any permanent improvements made over the years.
However, if he put them on the deed but retained a "life estate" -- the right to use the house until he died -- then that is usually treated as an inheritance, because the children did not have an unlimited right to use, enjoy, or sell the property until he died. In that case, their cost basis is the fair market value when he died.
(That's why giving a house to your children without proper legal and tax advice is always a bad idea.)
If you don't know the form of the deed, ask an attorney.
And, since it seems that your wife and sister in law have sold the house, once you figure out the cost basis, each sibling reports half the cost basis and half the sales proceeds, and pays tax on half the gain.
