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.
It's hard when all I can think about is being green with envy. What's wrong with me? I can't even get a date!
I can understand why you'd start to feel that way. I think it's good that you want to look inward instead of just blatantly blaming other people. That means you can start to look for ways to make you feel better about yourself, that are solutions! But nothing has to be "wrong with you." For some, it just takes time. For others, they find it easier to accept someone to settle down with.