Hi M.,
The measuring is not always accurate. I was measuring huge a few weeks before I was due, and my doc decided to induce, becuase she didn't want me to have to deliver a massive baby. It turns out that the baby wasn't huge at all, it was just so much amniotic fluid, and the position of the baby at the time. Your sil may just have a small baby right now, which is very common with firsts so early on in the pregnancy. I don't think she should worry. The docs are really great at calculating. My last preg, I didn't even know I was for a few months, and when I went in, the doc pegged my preg within a week from the actual (cuz I didn't remember). I don't think she should worry about the actual, as long as baby is ok. :o)
Mari