😠😠😠 Mother in law!

I'm almost 20 weeks pregnant with my first child and have my scan booked on Wednesday. We've said from the beginning that we don't want to find out the sex. So my mother in law has asked if we can get them to write it down put it in an envelope so she can know the sex so she can buy things!!! I mean how rude is that?! I want me and my partner to be the first to know not her. It'll ruin the experience for me knowing someone else knows. Plus if she let's slip to someone then that's it. What do you think? Am I overreacting? I'm not going to be emotionally blackmailed on this. My man clearly a mummy's boy said well she is buying us stuff for it.