Genesis 49:5

Shimeon and Levi are brothers of the womb; their thoughts are of sharp weapons for rapine.

SIMEON AND LEVI ARE BRETHREN. Jacob is saying that they possess the attribute of kinship for their hearts were inflamed concerning their sister., 34:25-26. He is thus stating in their defense that they acted as they did out of their brotherly zeal, thus suggesting [that were it not for this extenuating circumstance], they would have been deserving of great punishment and their sin would have been unforgivable since what they did to the people of Shechem was an act of violence.
The correct interpretation appears to me to be that he is saying that Simeon and Levi are real brothers, uniting in fraternity and brotherhood in counsel and deed. Now I have already explained, 34:13. that Jacob was angry with Simeon and Levi for having committed violence when they killed the people of the city of Shechem, for they had not sinned against them at all. They even made a covenant with them and they were circumcised, thus being enabled to return to G-d and become included within the people of the household of Abraham, part of the souls that they had gotten in Haran.” (Rashi, ibid.) Jacob was additionally angry with them lest people say that the matter was done at his suggestion, thus creating a profaning of G-d’s Name, as people will say that the prophet has committed violence and plunder. This is the intent of the verse, Let my soul not come into their council— this is an excuse for he was not in their council when they answered the people of Shechem with subtlety, and he was not united in their assembly when they came upon the people of the city and killed them. It was for this reason that he cursed their anger and wrath. And so did Onkelos translate: “My soul was not in their council when they assembled to strike, etc.”

מכירותיהם The reason why Yaakov used such a dual, pluralistic, mode of the word was the special relationship which existed between Shimon and Levi, a relationship almost like that of twin brothers, as if the term אחותיהם instead of merely אחים, would describe their mutual brotherliness. However, their special relationship appeared to concentrate on their doing something negative, i.e. they used destructive tools, weapons in the pursuit of their designs.

Whereas the terms מכורותיך and מולדתיך occur in Ezekiel 16,3-4, describing close relatives in one’s birthplace, in our verse, the vowel tzeyreh under the letter כ converts this word into one that could have been written with a dagesh, i.e. from the conjugation piel, the strong, transitive conjugation, suggesting that these emotions were evident only when the two brothers engaged in a scheme of doubtful ethical merit. The construction of the word is parallel to Job 37,12 where the word מסבות, describes clouds in constant motion foreboding turmoil, nature at its most destructive. Examples of the expected vowel chirik becoming a tzeyreh instead, are found in Lamentations 2,9, as well as in Numbers 8,26, and in Psalms 74,18. In all those examples the addition of the extra “dot” next to the chirik converts the meaning into something stronger, more aggressive, more intense.

אחים, seeing their older brother had been demoted, they would have been in line to have the honour due to a firstborn transferred to the brother next in seniority. However, they did not rate such special honour because

כלי חמס מכרותיהם, people who make use of weaponry in their day to day activities are not suitable material for wielding political and military power. Hence neither one of them will be accorded the status taken away from Reuven. (compare Proverbs 29,4 where Solomon describes a king’s primary characteristic as being במשפט יעמד ארץ, “seeing to it that the rule of law and justice prevails on earth.”) Shimon and Levi had disqualified themselves from claiming such a distinction.

שמעון ולוי אחים, they had acted like brothers in their concern for their sister Dinah, as the Torah credits them in Genesis 34,25.

כלי חמס, their swords, (weapons) have become instruments of violence. [used aggressively instead of defensively. Ed.] Yaakov considered the slaying of the male population of Shechem as having been illegal. He did not consider the men of the city responsible for the sin of rape committed by their crown prince Shechem. Seeing that their act was illegal they endangered their whole family, including their father, as they could not count on G’d’s protection against the avengers of Shechem when the wrong they had committed themselves had brought that attack upon them. The fact that in the event, G’d did protect them, was not thanks to these two brothers.

מכרותיהם, as if the Torah had written במכרותיהם. The construction is parallel to ארץ מגוריהם, the land of their sojourn, i.e. the land in which they sojourned. The people of Shechem had lived peaceably side by side with the family of Yaakov, there being no war-like actions between them. [The author apparently sees in the word מכרותיהם the root הכר, “to recognise someone, i.e. to know him well.” Yaakov describes the deed of Shimon and Levi against the people of Shechem as having been perpetrated on friendly unsuspecting neighbours. Ed.] Ezekiel 21,35 as well as 29,14 also speaks of מכורותיך meaning מגורים the place where one resides. Onkelos also translates it in this sense. Midrash Tanchuma Vayechi 9 understands the word as applying to their swords. In Greek a sword is known as makirin.

שמעון ולוי אחים, כלי חמס מכרותיהם, “Shimon and Levi are brothers, their kinship is the use they make of weapons.” Yaakov implied that whereas they had displayed their being brothers when avenging the rape of innocent Dinah, their sister, they had failed to do so in their treatment of Joseph, their brother. (B’reshit Rabbah,98,5 and 99,7)