I'd let her go to at least one of your scans....if you don't feel comfortable with her being at the "big" 20 week scan, invite her to another. Who knows, baby maybe not spread his/her legs at the 20 week scan and she may not miss out after all if she goes to a later scan?!

My mum came with me to my morphology scan with my DH. My sister came to an emergency u/s I has at 14 weeks due to bleeding. I'm very open to anyone and everyone sharing the experience lol