Reference for Makkot 26:3

ר' יצחק אומר חייבי כריתות בכלל היו ולמה יצאת כרת באחותו לדונו בכרת ולא במלקות

Rebbi Yitzchak says liabilities of kares are included [in the punishment of malkus], but why is kares singled out by [the prohibition of cohabiting] with his sister? [To teach us that liabilities of kares] are only subject to kares but not malkus.

Jastrow