I have a 1099S form for family land I sold to my brother. The land was originally deeded to my siblings and me without our knowledge.

[sturwar]

I don't have a date or fair mkt value for the transaction.

[Fern · Accepted solution]

You need to be able to determine a cost basis, otherwise 100% of the sales price will be considered your gain on the property.

Try working with your local taxing authority, whoever generates your property tax bill. They should be able to tell you when the land was transferred to you and your siblings. They may also be able to tell you what the assessed valuation of the property was at the time it was transferred.