"Mum, we don't want anyone else knowing the gender, it's our special surprise" and if she is offended bad luck. She'll forget it soon enough once she's holding grandbaby in her arms!
I totally agree. It's up to you who you tell, if anyone at all.

If she wants to watch the video and you're worried about the dialogue, just press the mute button.....make sure you hide the video away afterwards so she can't sneak a peek!