Are they really siblings?

We recently got the school health screenings for Tanner and Lindsey. Not surprisingly, they're both near the 50th percentile in height. It's the weight that's funny. Lindsey is in about the 75th percentile for weight. Tanner is in about the 5th percentile for weight. (His BMI is 14.0, which technically means he doesn't really exist.) It's hard to believe that those two are related, isn't it?