Parashas Acharei Mos · Passuk 18:17
Chabad.org — English translation + Rashi
Torah Temima points to Kerisos 15a.
English: “They are kindred; it is zimah” — the Torah treats them as one zimah: one act involving multiple overlapping relatives incurs a single liability.
Single zimah for overlapping relatives
When several forbidden relationships coincide in one act, the verse unifies them into one count of zimah.