Would you be annoyed?

My daughter (M) is 3 months older than her cousin (D)

My MIL was telling her MIL (and I quote) "M is 3 months older than D and D is SO much smarter than M"

...... Right in front of my face like I'm supposed to agree with her. She tells people this all the time and even tries to convince me.

M and D are on the level, sure D is a little more talkative than M (D went to daycare, M didn't)

I don't even know what to say in this situation. Any advice?