Your younger sibling is getting married before you, and everyone at the wedding knows it.

I can't believe the person whose diapers I used to change got married before me.

As if weddings weren't stressful enough for single people, adding the fact that your younger sibling is the one getting married pretty much breaks the dial on your brain's stress-ometer. Here is a rudimentary chart of wedding stress, based on personal data collection:

The biggest contributing factor to your need for a steady stream of Xanax is how everyone else at the wedding treats you. They ask all the questions that you've already asked yourself, and point out all the things that you're trying to shove deep into the recesses of your conscious mind.