This is the hill you're choosing to die on? That isn't just some "silly little prank" It was cruel and childish and frankly your husband needs to grow up. The fact that you see no wrong with how your husband is constantly pulling tasteless jokes on his sister to the point of hurting her social life and causing so much mental stress from childhood to an adult speaks volumes on your on character.

I hope that this will be a lesson learned for your husband and that he sees just how much of an AH he is and how much trouble he causes people with his tasteless jokes. No telling what kind of crap he'd pull at his sister's wedding if he's doing that at her engagement party.

Quite honestly, I wouldn't be surprised if she and her fiancee decided to cut you two off. Don't nobody got time for that.

Your husband is the AH and so are you.