In Vayechi, the Torah states, "Shimon ViLevi Achim” “Shimon and Levi are brothers (Bereishit 49:5).” When Yaakov rebuked Shimon and Levi for being violent, he called them "Achim,” “brothers.” It seems that Yaakov was grouping Shimon and Levi together, but this appears difficult considering how far Shimon and Levi separated from each other over a long period of time. For instance, Moshe, Aharon, and Miriam, three of the leaders of Klal Yisrael throughout the Torah, came from Sheivet Levi, while the only personality of note emerging from Sheivet Shimon was Zimri Ben Salu and the 24,000 people that were killed along with him. In addition, both Shimon and Levi were two of the smallest Shevatim. However, Levi was small because they had the dangerous job of carrying the Aron. Alternatively, according to the Ramban, they were small because they were not part of the enslavement in Mitzraim, and therefore did not multiply like the rest of Bnei Yisrael. As the Torah states, "Vichasher Yaanu Oto Ken Yarbeh Vichen Yifrotz" “And just how they inflicted them that’s how much they multiplied and became fruitful (Shemot 1:12).” Shevet Shimon was small because of the many of them died in the incident with Zimri. Shimon also remained small because they were not given land like the rest of the Shevatim, as it states in Sefer Yehoshua "VaYihi Nachalatam BiToch Nachalat Bnei Yehuda" “And their portion was in the portion of Yehudah (Shemot 1:12).” Although Levi was also not given their own specific land, this is due to their holiness and their requirement to work in the Mishkan, as it states in Parashat Shofetim, "Hashem Who Nachalatam" “Hashem is their portion (Devarim 18:2).”
In his Sefer MeiEin Beit HaShoeiva, Rav Shimon Schwab offers an answer to the problem of Yaakov grouping Shimon and Levi together despite their extreme differences. He answers that while both of the brothers heard the "Musar" from their father, Levi took his father’s words to heart and tried to change his qualities for the better based on his father’s rebuke. On the other hand, Shimon did not try as hard to change. Levi strived to grow in Torah and Mitzvot, and as a result, he merited a greater reward in the end. The difference between Shimon and Levi came full circle in Parashat Pinchas when Pinchas, a member of Shevet Levi and who was zealous for the sake of Hashem, had to kill the head of Shevet Shimon, Zimri, who represented how Shimon did not do Teshuva like Levi.
Although Yaakov grouped Shimon and Levi together when rebuking them, only Levi did Teshuvah as a result of Yaakov's words. Over time, the two brothers separated from each other in both values and goals. We can learn from the idea presented by Rav Schwab how important it is for one to learn from the criticism and Musar that he receives. One should try his best to change his ways for the better. With the help of Hashem, we will take this challenge that RavSchwab presents seriously, and we will choose to follow the path of Torah and Mitzvot like Levi.