Normally, every family expects the older daughter or son to get married first. In most Jewish communities this is the accepted Minhag (=tradition). There are two sources for it. One is an allusion (remez) to this ancient custom from the words of Laban to Ya'aqob, when Laban refused to give his younger daughter Rachel in marriage before Leah. He said: "In our place, this will not be done, giving the younger (to marriage) before the older" (Bereshit 29:26). The second and more established source is the case of the five daughters of Tzelofchad, who according to our rabbis got married in the order of their age.
Nevertheless, the Rabbis agree that this Minhag establishes a preference for the older sibling to get married first, but not a prohibition for the younger sister to get married, if she is at the age of marriage.
Rabbi Moshe Feinstein, held a very interesting interpretation of this Minhag. In his opinion (IGM, EH'E 2:1) from the two above mentioned cases we could deduce that giving the priority to the older sibling to get married applies only when both of them are engaged. Then, the wedding of the older sister or brother must take place first. But other than, if for whatever reason the younger sibling finds her spouse first, she should not be prevented from the opportunity.
In many communities it is customary that in this case, the younger brother or sister asks formally permission from his older sibling (let us not forget that part of the Mitzva of Kibbud Ab va-Em includes also the respect for the older brothers or sisters. See more here) and the older sibling gives his or her blessing wholeheartedly.
READ: "That Sinking Feeling"The tragedy of the Costa Concordia.
by Rabbi Benjamin Blech