With the will or grant of probate, the estate accounts, the executor's record of distribution, evidence of where the deceased's own funds lawfully came from where required, and bank records showing the money reaching the investor.
An inheritance needs the estate's story as well as your own
Hypothetical example: the family's funds come partly from a parent's estate settled three years ago. Documenting the receipt is straightforward; documenting the origin sometimes is not, because the question can extend to how the deceased lawfully accumulated the assets. Gather the will or grant, the estate inventory and accounts, the executor's distribution statement, and any tax clearance obtained.
Where the estate held a business or property, include the records showing what was sold and for how much. Then the receipt: the deposit into the investor's account and every subsequent transfer. Do the same for the duplex sale in a parallel file.
Where the two sources were mixed in one account, provide statements that let a reader follow each amount, and prepare a short written narrative with document references so nobody has to reconstruct it.