What evidence or proof does the governing body used that they are chosen from God?
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No evidence exists. It is based solely on their say so. Conveniently, everything involving the alleged events of 1919 happened "invisibly".
My suggestion: why not ask your mother what one thing would cause her to doubt her beliefs or her religion is the true one? Whatever the answer is, that's what you discuss. Everyone has their "hot button", and trying to guess what that is by randomly choosing issues to dismantle may be a complete waste of effort and further entrench her into defending WT.
[edit] Ding, you got there right before me. Great minds....