Most ponies don't know every single detail about their friends. And even when certain details are known, they usually don't bother mentioning them without rhyme or reason. For example, Rainbow Dash and Fluttershy both know each others' families pretty closely, but nopony's looking for an excuse to remind themselves that Zephyr Breeze is a thing.

Conversely, when your friends first find out about a heretofore-unmentioned sibling, they will probably want details. So when Twilight receives an invitation to her brother's wedding, her friends are naturally curious about him.

Turns out, there's a good reason why she doesn't talk about him.

Written for F*** THIS PROMPT #14.