She insists on inappropriate underwear at wedding

DEAR ABBY: I am getting married, and my fiance’s daughters, 19 and 21, are in the wedding party. I have purchased the dresses they are wearing, which are light and flowing.

Jeanne Phillips 

I have told the girls that on the day of the wedding I do not want them wearing thong underwear. The older one then went to her dad and said she didn’t want to wear regular underwear. He told her she could wear whatever she wants.

I have tried telling them that as young ladies there are times you don’t wear thongs, and under a flowing dress is one of them.

It’s one day of their lives. How can I get my point across?