Is it wrong to be irritated by my sister's pregnancy?

This just sounds awful to me. You and your husband made it somehow so may be she thinks she can do it too. Although really, why have a baby when you are 18 and have no life whatsoever. You guys should have advised her to not keep the baby, if you could. But since it is on the way, there is nothing you can do but try to be more understanding but firm and show her that this is a huge responsibility she has to cope with together with the father. Good luck!