It's a complex issue, but think of it like backwards fundamentalist Christians going ballistic over women (especially their daughters) wearing anything more revealing than a sweater. That kind of familial coercion is couched in religious terms and springs from religious values, but is not, strictly speaking, fundamental (ha) to the religion itself.

The root of the thing is that Islamic teachings require practitioners to dress modestly.

Interpretations of this range from "cover your sex bits" to "don't show an inch of skin to anybody except your husband".