13 month old afraid of my mom, and only my mom

I have two kids, 3.5y and 13m. My 3.5 has loved my mom since the day he came out. They have a tight bond and it’s effortless. He is not a fan of most people and is kind of a grump. She’s really good with him and is the only person we trust to babysit overnight. My 13 month old however is a super happy smiley and social baby. He has never liked her. Even as a little baby he refused to smile for her, and as he’s gotten older he cries when she talks to him. The other day my older child was FaceTiming her and he turned the camera so she said hi and he immediately started bawling.

We are supposed to have a date night this weekend (5 days from now) where she watches them. Usually it’s her and my step dad, and he adores my step dad. But he will be out of town, and I didn’t know that. I thought about maybe telling her to give him a bath (that’s his FAVORITE activity).

Any ideas why or how to help this?

I will try to be active to answer clarifying questions.