What to say to my sister in law and brother who's baby has died at 2 days old and only 14 ounces

About all you can say is that you are so sorry and that you are there for them if they ever feel the need to talk. Don't think you will hurt them by talking about their loss... or mentioning it at a later time.

Many people, in an effort to help the family avoid additional pain, will not talk about it; almost ignoring the situation. This often has the opposite effect... causing some people to feel that if they do have the need to talk, they fear they will make everyone else uncomfortable or that perhaps others don't find it important enough to bring up.