How does a Trent Lakes family document money earned over thirty years of land sales?

By rebuilding the chain from public and institutional records rather than from recollection. Registered transfers, statements of adjustments, mortgage discharges, filed tax returns and historic bank statements obtained from successor institutions do the work. Affidavits help only where they sit alongside surviving documents.

Start at the registry, not at the memory

Land is the easiest asset class to evidence because most of what happened was registered. For each parcel, obtain the instrument that conveyed it in and the instrument that conveyed it out, the consideration shown, and the statement of adjustments that reconciles the price with what actually reached the vendor. Match each of those to the tax return that reported the disposition, because a gain declared to a tax authority is powerful corroboration.

Then follow the proceeds: request historic statements from the bank that received them, and where the institution has been amalgamated, ask its successor, which usually holds the archive. For the inherited portion, add the will, the grant, and the estate accounting that shows what passed to Sunniva. Gaps will remain, and they should be identified and explained in a written narrative rather than hidden.

What defeats these files is not an old transaction but an unexplained transfer between accounts, a cash component nobody documented, or a loan from a relative that appears in the trail with no agreement behind it.