If a young man is suggested a Shidduch with a girl who is, let's say, 21 years old, and that girl has an unmarried 23-year-old sister, would it be improper for the boy to date the girl? Should he be concerned about the possibility that the older sister will feel resentment over the fact that her younger sister is marrying before her, which could invite the "Ayin Ha'ra" (evil eye) or otherwise cast a dark shadow over the marriage? This question also arises in the reverse case, of a girl who is introduced to a young man who has an older unmarried brother. This question was addressed already by the Maharit (Rav Yosef of Trani, 1568-1639), who wrote that there is no prohibition at all against marrying somebody who has an older unmarried sibling. He notes that this concern was relevant only in ancient times when it was customary for the father to marry off his daughters when they were still children, by accepting Kiddushin on their behalf. When the father accepts Kiddushin for the betrothal of his daughters, it is improper to marry off his younger daughter before the older daughter. Nowadays, of course, young women marry on their own, when they are adults, and in such a case there is no concern whatsoever when a younger sister marries before her older sister. Similarly, the Or Ha'haim (Torah commentary by Rav Haim Ben-Attar, 1696-1743), in Parashat Vayeseh (29:26), writes that the sequence of marriage is of concern only for the girls' father, but not for the groom. Meaning, if a man wishes to marry a younger sister, he may certainly do so, and it is the sisters' father who must worry about first marrying off the older daughter. The Or Ha'haim draws proof to this theory from Yaakob Abinu, who wished to marry Rahel even though her older sister, Leah, was unmarried. Of course, Laban switched the two sisters, but regardless, Yaakob was prepared to marry Rahel even though Leah was unmarried, proving that this is not an issue with which a groom must be concerned. Rav Yosef Shalom Elyashiv (contemporary) indeed rules that one may date and marry a girl who has an older unmarried sister, and a girl may marry a boy with an older unmarried brother, as the older sibling's need to get married is not the responsibility of the younger sibling's suitor. And besides, as Rav Elyashiv noted, it can generally be assumed that the older sibling does not mind the younger sibling's marriage. Certainly, a younger sibling should not be forced to remain single just because he or she has an unmarried older sibling. Therefore, an older unmarried sibling should not be a factor in considering a marriage prospect. Ideally, of course, we want all our children to marry as soon as they are ready and in order, but if this does not happen, the younger siblings are free to date and marry even if there is an unmarried older sibling. Summary: It is entirely permissible for a girl to date and marry a boy who has an older unmarried brother, and for a boy to date and marry a girl who has an older unmarried sister.