I know for a fact my aunt mails in ballots for her husband, her father and her mother. Claiming that there is no mail-in voter fraud among 300 million Americans is quite absurd in my opinion.
It's a bit absurd when claims of rampant mail-in voter fraud come without evidence from a New Yorker in the District of Columbia who votes in Florida by mail.
One, why haven't you reported your aunt for voter fraud? Voter fraud is a very serious issue and it seems to me that you're letting a criminal go free. This can be easily verified by telling the government and having the three others confirm who they voted for.
For two, if she is mailing in the ballots, then is she forging their signature as well as collecting their SSNs? Or are they simply signing on the ballots themselves agreeing to the votes? In which case is that truly fraud or not? If she's forging signatures then she's also committing identity theft which is a very serious issue, one that I hope you agree deserves reporting.
That said, perhaps “unsubstantiated” is a better description.