just be polite and firm telling her your choice - and you know what u do, you are NOT open for opinion. let her be if she not happy, she should know her place and your husband should talk to her separately too.
try speaking to your husband about your concerns. For me I couldn't breastfeed due to long term medication which will affect my boy so we went for formula milk and everything is fine.
My MiL snatches to feed babies so she would rather I not breastfeed! But at the same time nags that I dun have milk when she literally stands in front of me to see me feed 😴
Talk to your husband and ask him to manage his mom