I unfortunately agree with those that say to wait awhile. Right now, all attention is being showered on your sister, and her wedding. Pretty sure they would have further feelings of "you ruined the wedding" if you tell them now.
Just a side note. I know many people have somewhat different views on when to marry these days, and I will freely admit that I am "old school" on this one. One thing to consider, though. If something happens to your health, the baby's father is NOT legally able to make decisions for you, and your family may be called upon to do that. There are many advantages to being legally married.
Also, once you've been living together and a baby is on the way, the large wedding thing isn't going to be looked on the same way. A small wedding with those that truly love you attending can be beautiful and personal.
hope the best for you and baby.