8 week ultrasound.... Still be twins?

My sister is driving me crazy. She is due in November. Her insurance is messed up so hasnt been since she was 8 weeks pregnant. She had her ultrasound at 8 weeks. Saw one baby and one heartbeat. But she is still convinced she is having twins. She isnt very big, but says she felt movement since 13 weeks and she is currenly 20 weeks. Says she feels movement way above her belly on now. So my question is could she still have twins even though the first ultrasound only showed one???