It sounds crazy but I get it. Might not have anything to do with her not trusting or thinking the baby's father sickly gets down like that.
It could be that she was taught or told by other women in her family to watch over her daughter when it comes to bathing, changing diapers, etc y'know let the women attend to the girls and let the men of the family attend to the boys.
If this was a baby boy, she'd probably be okay with it because, y'know, he's a man, and that's his son, but with a daughter, I could understand why she feels the need to be the only one washing and changing her diapers.