My mum guessed (i got twnety questions when i was quiet on the phone) so I'm not sure
Do you plan on telling her over the phone or in person?
Do you want it to come across as scared and fearful or would you like to feel confident about it?

If you wangt to show her from the start that you're happy and you have made this decision consciously (rather than fearing what she might say) you could buy her a fridge magnet or something that says "What happens at Grandma's stays at Grandma's" Do your brothers have any children or is the the first grandchild?

Or just come out with it?

Sorry i don't have any better advice, good luck!