Oral will need to be proven in court and unless your FIL has committed to give your wife the entire 7 acres in front of witnesses who are ready to testify in the court, it might be a tad difficult to prove the same.
Your wife will any way get 2 acres out of the 7 acres as your father in law did not make a Will with respect to these 7 acres. so these 7 acres need to be distributed equally amongst the legal heirs .
Legally the Debts need to be paid out first by selling properties and then balance distributed by the properties. So ask for partition of properties and also statement of debts.
Any debt that is predated prior to partition of properties (15 acres to BIL, 10 acres to your SIL, 4acres to your wife) should be paid out from their(BIL, SIL) respective shares.
Any debt that is ante dated subsequent to partition of properties, should be settled from the 7 acres of remaining properties.
With reference to title deeds, certified copies of title deeds can be obtained at Sub registrar office as these are public documents ( you can get certified copies of title deeds if you know the survey number, extent of land etc)
Hope this helps