I agree with Kelly, it's not the sonographer's job to comment, and people can get really upset if they are wrong, so they're better off not commenting so they don't get in trouble!

They might say 'ooh that's not good' and then the mum is upset and everything might actually be fine, or they might say 'everything looks great' and then if it turns out there's a problem, the mum is upset because things weren't fine.
So it's probably not a bad policy for sonographers just to let the doctor do the talking

Not sure if it's worth telling your friend's sister not to worry at all (because that seems to be impossible for expectant mums!!), but it might ease her mind to consider that not commenting is just the sonographer's standard policy.