Our Gemara on Amud Aleph (and the previous Amud) takes a positive spin on the incestuous actions of Lot’s daughters. From their perspective, seeing the destruction of Sodom, they thought the world had come to an end. They saw it as their duty to repopulate the earth even if it meant having relations with their father. As such, since it was in their mind a mitzvah, so it was. (See yesterday’s Psychology of the Daf, Nazir 23.)
אָמַר רַבִּי חִיָּיא בַּר אָבִין, אָמַר רַבִּי יְהוֹשֻׁעַ בֶּן קׇרְחָה: לְעוֹלָם יַקְדִּים אָדָם לִדְבַר מִצְוָה, שֶׁבִּשְׂכַר לַיְלָה אַחַת שֶׁקְּדָמַתָּה בְּכִירָה לִצְעִירָה זָכְתָה וְקָדְמָה אַרְבָּעָה דּוֹרוֹת בְּיִשְׂרָאֵל לַמַּלְכוּת.
Rabbi Ḥiyya bar Avin said that Rabbi Yehoshua ben Korḥa said: A person should always come first with regard to a matter of a mitzva, as in reward of the one night that the elder daughter of Lot preceded the younger for the sake of a mitzva, she merited to precede the younger daughter by four generations to the monarchy of the Jewish people. The descendants of Ruth the Moabite ruled over the Jewish people for four generations: Obed, Yishai, David, and Solomon, before the reign of Solomon’s son Rehoboam, whose mother was Naamah the Ammonite.
I think it is also notable that the Gemara praises the oldest for taking action first. You might have thought that the youngest should have been expected to wait her turn, out of respect for her older sister. But if that were true, she would not have suffered any consequences for having been second, since it was an appropriate delay. From here we see that when it comes to “chapping a mitzvah”, certain courtesies may not apply.
It is still forbidden to steal someone’s mitzvah, see Bava Kama 91b where someone had to pay a large fine for stealing the other person’s mitzvah of covering the blood of a slaughtered bird. But that is in regard to a mitzvah someone intends to do and is incumbent specifically upon him, and you took away the opportunity by grabbing it first. It is not in relation to a mitzvah equally available and obligated by both people.
There is a strong custom amongst certain Jews that we do not marry outside of the birth order, and the oldest goes first. The Bach (Tur YD 244:14) seems to rule that a younger sibling should not marry before an older sibling, citing both the practices of the daughters of Tzelafechad, who married in birth order (Bava Basra 120a and Rashbam ibid), as well as Lavan, who said,
וַיֹּ֣אמֶר לָבָ֔ן לֹא־יֵעָשֶׂ֥ה כֵ֖ן בִּמְקוֹמֵ֑נוּ לָתֵ֥ת הַצְּעִירָ֖ה לִפְנֵ֥י הַבְּכִירָֽה׃
Laban said, “It is not the practice in our place to marry off the younger before the older.” (Bereishis 29:26).
Lavan may have been a trickster, but we see he followed ancient proto-Jewish traditions. He also practiced Sheva Berachos (ibid 27).
Rav Moshe (EH II:1) rules that this restriction only applies when both have the ability to marry and it is merely a matter of whose wedding comes first. This is not merely being of marriageable age, but to actually have a confirmed shidduch and ready to set the wedding day. However, Rav Moshe asks rhetorically, “Can we expect the younger child to delay a mitzvah if the older sibling is not ready?”
I believe my diyyuk in the Gemara is a support to Rav Moshe’s position. If the younger is not expected to wait on performance of the dubious aveira lishma (sin with good intentions) of the daughters of Lot, Kal V’chomer a regular sibling should not have to delay performing the mitzvah of marriage.