Gfyuki

Rashi brings the Midrash that Yocheved was born as Yaakov's family arrived in Egypt. They had to migrate due to the devastating famine.

Now, Rashi earlier points out that we learn from Yosef that it's forbidden to be intimate with one's wife during a famine. This is learned out from the fact that the Torah stresses that Efraim and Menashe were born before the famine started. This halacha is codified in Shulchan Aruch (Orach Chaim 240:12).

How then was Yocheved conceived by Levi during the famine? I admit I'm assuming he kept this halacha just like Yosef, as I see no reason why Yosef would follow it and not the rest of his family. Otherwise how could we learn anything from Yosef's behavior; maybe he was just being extra pious.

Now, the Shulchan Aruch brings a special dispensation for those who have not yet fulfilled pru urvu, having a boy and girl. However, I don't see how this helps, as Yosef also didn't have a boy and girl, yet the Torah stresses that he abstained from his wife.

I just discovered that many Rishonim address this question. For example:

Tosafos to Taanis 11a s.v. אסור לאדם

אסור לאדם לשמש מטתו בשני רעבון - ואם תאמר הרי יוכבד נולדה בין החומות ואותו העת עת רעב היה וע"כ שימשו מטותיהן בשני רעבון ויש לומר דלכ"ע לא הוי אסור אלא למי שרוצה לנהוג עצמו בחסידות ויוסף לא שימש אבל שאר בני אדם שימשו

If you ask, Yocheved, who was born between the walls, and at that time it was a famine, so it must be they were intimate during the famine...an answer is that it's not prohibited for everyone, just those who want to act with extra piety. Therefore Yosef was abstinent but not others.

Chizkuni to Genesis 41:50

י"ל דיוסף היה לו לפרוש מתשמיש המטה לפי שאביו ואחיו שרויין בצער אבל לוי שהיה יודע שאין מחסור לאביו לא היה לו לפרוש, ואם בשביל יוסף לא אסרו הדבר בשביל היחיד.

You can say that Yosef should have been abstinent since his father and brothers were suffering in the famine. Levi however knew that his family wasn't lacking, so he didn't need to abstain. If [you'll ask] he should have abstained because of [the pain of] Yosef, the matter isn't forbidden just because of an individual.

The Chizkuni seems to understand the prohibition relates to having family members suffer. Not sure where he got that. However, Rav Chaim Paltiel gives the same answer in the name of Rabbeinu Elchanan

Daas Zekeinim ad. loc.

ואמר ר' יהודה החסיד דמה שאסור לשמש מטתו כגון נביא ששמע מפי הקדוש ברוך הוא שעתיד להיות רעב כגון יוסף שידע הדבר בבירור על ידי חלום פרעה אבל לוי שלא היה יודע הדבר בבירור לא היה נאסר לו

Rabbi Yehudah HaChossid answers that it's forbidden only for a prophet who is told in prophecy that there will be a famine, like Yosef, who knew through the dream of Pharaoh. Levi however didn't know as clearly, so intimacy wasn't forbidden.

They give a second answer:

ויש מתרצין דלוי ויוסף פליגי בפלוגתא דתנאי דפליגי במסכת יבמות דאיכא מאן דאמר התם מצות פריה ורביה שני זכרים ואיכא מאן דאמר דלא קיים עד שיוליד זכר ונקבה. ולוי סבר כמ"ד עד שיוליד נקבה ג"כ ולפיכך שמש כי עדיין לא נולדה לו נקבה ואמרינן במס' תענית חשוכי בנים משמשין מטותיהן בשנ' רעבון כדי לקיים מצות פריה ורביה. ויוסף ס"ל כמ"ד שני זכרים ולפיכך לא רצה לשמש כי כבר קיים

Some answer that Levi and Yosef disagree how many children are needed to fulfill pru urvu. Levi held that until you have a boy and a girl, so he continued to be with his wife during the famine as the gemarra says those without children are permitted to be together during a famine. Yosef however held that you've fulfilled the mitzvah with two boys, so he didn't want to further be with his wife once he fulfilled the mitzvah.

Yaakov's family wasn't subject to the famine. He had plenty of money and was in the process of purchasing grain, which was for sale to the public, from Egypt when Joseph revealed himself. Perhaps if he had no more money or grain was no longer for sale it would have been forbidden.

Furthermore, there are commentators who posit that Yaakov miraculously had plenty of food and only sent the brothers to purchase food so neighbors won't be covetous of him.