Sister in law not happy...

I know that my sister & brother in law have been trying for a while to get pregnant with no luck. So yesterday, my husband and I announced that we're pregnant with #2. They got up and left in the middle of dinner. They said that she was sick and not feeling well before we got there but I know that she is upset about us being pregnant. How do I help her understand that while she doesn't have to be happy for us, our intention wasn't to upset her. Or how do I not feel terrible about what happened? My husband and I really feel horrible that we upset them like that!