Is she pregnant?

Tiffany • Im 23, married to a wonderful man, have a beautiful 4 year old daughter, and currently in Mexico. 🇲🇽🇲🇽🇲🇽
So about 5 weeks ago, my sister-in-law found out she was 3 weeks pregnant. She should be around 8 weeks about now. She went to her first appointment for a confirmation ultrasound. The doctor told her the sac is empty and maybe she's earlier then she thought. 
I'm wondering if she should be 8 weeks now, shouldn't you see a baby on the ultrasound? Or could she have only been two weeks when she found out she was pregnant, wouldn't a baby show on a ultrasound at 6-7 weeks? She says she's pregnant, she's got the symptoms. I told her to schedule another ultrasound in 2-3 weeks to see if there's a baby. She said she doesn't need one because she knows she's pregnant. I told her that sometimes you can have pregnancy symptoms and not be pregnant. She still doesn't want one.  Could she still be pregnant or could this be possibly a blighted ovum? 
Any answers will be appreciated. 
Thanks a bunch.