Let's hope they won't kill the messenger.

make them tell their parents themselves.

Well, clearly neither of them were mature enough to be married in the first place, let alone to each other. Tell them to man and woman up and tell your parents themselves, it's not your divorce.

I'm so curious to find out how the hell did they play it off over the holidays and birthdays? Even if they lived too far away for face-to-face visits, wouldn't the folks have at least called to say Merry Christmas to both of them?

Wow. You should tell your brother and ex-sister-in-law that you tried telling your parents, but that they didn't believe you. That way, the two of them would be forced to tell the truth themselves ... or pretend to be married for the rest of their lives if they are really so immature!

Tell her that they already know

Don't say anything, it would be funny to see your brother explain at holidays where his wife is, and who the new side piece is.

I'm not sure I understand why this is something to be afraid to tell someone. It's not like your parents can punish them for the fact that their marriage fell through.

I hope you were disinclined to acquiesce to their request. (it means no)

Tell your brother that you'll be wingman and be there when he tells them, but he needs to tell them.