Musar for Shabbat 236:7

This also explains the Midrash saying that Tzipporah was so named, since like a bird, צפור, she purified, the reference being to the offerings of צפורים to signify purification of people who had previously been afflicted with צרעת (Shemot Rabbah 32; Leviticus 14,4). Similarly it is used to purify the house afflicted with certain kinds of stains (Leviticus 14,49). Rabbi Yossi used to refer to his wife as "his house". As long as this twin sister of Abel had been under the control of Cain,- maybe she (Tzipporah) blamed G–d. Cain was jealous, blaming his fate on the original sin of his father Adam who had brought contamination into the world, had made him an outcast like the מצרע the person afflicted with that skin disease. This fits because we know that such an affliction is usually due to slanderous talk of a person (Eyruvin 15). Moses' staff, מטה, was rooted in the עץ החיים, tree of life, the סוד of Abel who had not been tainted with sin. Abel lived for 49 days as the Ari zal explains the meaning of the word שבעתים "sevenfold," to mean seven times seven (Genesis 4,15). The word יקם, "will be avenged," is composed of the respective letters in the threesome יתרו-קין-מצרי. Abel lived 49 days in this world which was created with the letter "ה" (Genesis 2,4, see Menachot 29). The number 49 is equivalent to the numerical value of the word מט"ה, Moses' staff. The letter ה in the word is a reference only to G–d having created the world with the help of that letter. It became the staff and symbol of Moses later on. When Moses refused the mission to go to Pharaoh and bring about the חידוש העולם, his staff was turned into a צרעת serpent, and his hand became afflicted with צרעת, to show that he had aborted his purpose in life.