Why does the Torah repeat the Isur twice (here and in 18:13)?
Rashi: Because one is Chayav on both the sister of one's father and of one's mother, whether she is a paternal sister or a maternal sister. 1
Rashi: As opposed to Isur of one's uncle's wife (in the following Pasuk), which applies only to the wife of his father's paternal brother.
Why is the punishment for an aunt left vague ("Avonam Yisa'u"), and there is no verse at all about the punishment for Achos Ishto? A punishment is written for all the other Arayos!
Riva (10, citing Ibn Ezra and R"M of Kutzi), Moshav Zekenim (18:25): Moshe did not want to write them, due to the honor of his father Amram 1 , who married his father's sister 2 , and the honor of Yakov, who married two sisters. Since he did not write about a father's sister, he omitted also a mother's sister.. 3 (There is Kares for all the Arayos, due to the Hekesh of 18:29.)
Moshav Zekenim: Moshe taught all the Arayos to the Nesi'im and the Zekenim briefly (for we do not expound about Arayos to three people - Chagigah 2:1), and they taught Yisrael (i.e. everyone taught at most two Talmidim at a time. Moshav Zekenim also asked why the Torah omitted "Ish", which it says about all other Arayos. Perhaps it is to avoid calling attention to Amram, who is called "Ish"(Shemos 2:1). -PF)
Sifsei Chachamim (Shemos 6:20): The verse calls Yocheved "Dodaso" of Amram. She was his father's sister. In Parshas Arayos, Dodaso is the uncle's wife.
The Torah could have taught a punishment for the mother's sister alone, just like a separate verse forbids her (18:13)! - PF)