Is a wife sinful if she asks her husband to postpone the obligatory Hajj, fearing that she might not find a mahram in the future?

Hajj is an immediate obligation for whoever can find a way to it, and delaying it is forbidden. If the Hajj lottery selects the husband, and he is able, then Hajj becomes obligatory for him, and it is forbidden for him to delay it. The wife should not ask him to wait for her, especially since the lottery system in Libya allows a woman to be accompanied by her mahram when she is selected.