How can one subpoena for bank records to clarify inheritance?

Lifetime gifts which are intended to reduce the recipient's inheritance must be acknowledged in writing by the giver or the recipient at the time the gift is made. So if your parents didn't write it down or you and your siblings didn't write it down back when the gifts were first made, then those gifts don't reduce any of your inheritances. Usually, when someone gives a gift which is intended to reduce an inheritance, they contact their attorney and get it written formally as a codicil to their will. If it was written at the time of the gift, then the written acknowledgement would be the evidence of the amount of the gift - not bank statements. If your parent's wrote - "We give our son Tommie $500,000 in cash which will be considered an advancement of his inheritance" then his inheritance would be reduced by $500,000 even if the bank account shows that he was paid $300,000 or $600,000. It doesn't sound as if your parents wrote these things down, so all children would share equally in the inheritance at this point.