Negative Commandment #129, 130
Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
Second Section – the Negative Commandments
129 & 130. It is a negative commandment "do not have relations with one's father's sister"
as it states in Vayikra (Leviticus) 18:12, "you shall not uncover the nakedness of your father's sister."
And it is a negative commandment "do not have relations with one's mother's sister"
as it states in Vayikra 18:13, "you shall not uncover the nakedness of your mother's sister." There is no difference if she is the sister from the father or from the mother of his father or mother, even if she is the sister through an immoral relation, in all of these cases she is still considered his father's sister or his mother's sister. The punishment for having relations with either his father's sister or his mother's sister is: if it was done with intent, he is spiritually excommunicated - this is true even if there were witnesses and warning. If it was done by accident, a sin offering is required.
This applies in all places and at all times.