I wonder what makes her cry?

perhaps she is reminded too much of her father when his brother holds her and she doesn't quite know what to make of him. that's the only thing that comes to mind. I wouldn't worry. Let him talk to her and play with her from a distance and get to know her gradually. soon she will be old enough to initiate contact (when she can crawl over) and just let her control the interaction. I'm sure she'll get over it.