Your sister and her husband needs to tell that elder off.
If he's gonna pull rules out of his ass he should be told to stuff them back up there
This is ONE day, a special day, it is for families and friends. It is NOT a KINGDOM hall!
And she's not even baptized - NO elders have any jurisdiction over her. Tell her to talk to another elder.
They will confirm that the one elder is an idiot and has no right to be dictating terms to a non-baptized individual.
This makes me so mad