["My younger sister is getting married before I am, I can't believe it."]
Do you feel negative about it, or positive? It's hard to tell from your sentence. Maybe you feel both ways. If I were in your shoes, I could see how you would start to feel some more pressure in that situation. I was always the youngest child, so I never had pressure on me to accomplish anything by a certain time. If you're not happy about it, hopefully you can turn it into a positive so that your sister doesn't get any undue negativity.
My younger sister is getting married before I am.