She went to the doctor the other day and had an ultrasound. They said they could not see anything except for the sac for her to come back in 2 weeks and if they still couldn't see anything that something was wrong. My sister is really worried about this. Is any of this true?

It is very early. It is definitely possible that they were unable to see anything on the u/s. in 2 weeks time they should be able to see at least the yolk sac and most likely the baby. I mean we are talking about something that is basically microscopic at this point. This is not uncommon. I woiuld not worry too much about it.

I had a ultrasound done at 4 weeks and they said they were no sighn of even being pregnant but at 6 weeks I had one and they seen the baby everything and I am now almost 25 weeks with a healthy baby girl so tell her good luck and not to worry too much