Salam masnoon, is it lawful to delay the marriage of a younger daughter (despite a suitable proposal) till the marriage of the elder daughter (who is still awaiting a proposal)?
There is no reason to delay the marriage of one daughter when a suitable proposal is available only because her elder sibling is not yet married. These kind of practices are cultural and have no basis in the teachings of Islam. One should avoid following such cultural practices when they in contrast with the pure teachings of Islam.
And Allah knows best.
Mufti Ikram ul Haq
Fatwa Center of America