your dh should've kept his mouth shut. my mil is a bit of a blabber mouth, not maliciously, she just struggles to keep big news to herself. knowing this, we both agreed no news would be told to anyone until we were both ready to have everyone know.
it's annoying he thought he could tell her though and she's not managed to keep it quiet for your sakes. I'm sorry it ended up in a fight between you and your dh too!