TWINS?!

So my sister is 5 weeks 5 days pregnant & when she went to her appt the doctor said her uterus was measuring bigger. She's very smalk, maybe 110 pounds. What could this mean?? There's no possible way that she is farther along than 6 weeks because the farther was the 1st guy she had had sex with in 5 months. & apparently she got pregnant the very first time they had sex. No judging! I'm hoping it means twins :) Anyone else go through this? What did it mean?