My younger sister is pregnant...

I feel like this is going to sound weird, but my little sister is pregnant and I don't know how to feel about it. She is 20 and I am 24. I just don't know how to feel about it. I guess I just assumed that I would have children first because I am the oldest of 4 sisters. I'm not ready to have children any time soon, nor do I want to. I just feel like I'm more upset than I should be about her pregnancy and I don't know why. Any advice?