sa uttarasya tanayam
tasyam utpadayat sutan
sah—he; uttarasya—of King Uttara; tanayam—daughter; upayeme—married; iravatim—Iravati; janamejaya-adin—headed by Maharaja Janamejaya; caturah—four; tasyam—in her; utpadayat—begot; sutan—sons.
King Pariksit married the daughter of King Uttara and begot four sons, headed by Maharaja Janamejaya.
Maharaja Uttara was the son of Virata and maternal uncle of Maharaja Pariksit. Iravati, being the daughter of Maharaja Uttara, was the cousin-sister of Maharaja Pariksit, but cousin-brothers and sisters were allowed to get married if they did not belong to the same gotra, or family. In the Vedic system of marriage, the importance of the gotra, or family, was stressed. Arjuna also married Subhadra, although she was his maternal cousin-sister.
Janamejaya: One of the rajarsi kings and the famous son of Maharaja Pariksit. His mother's name was Iravati, or according to some, Madravati. Maharaja Janamejaya begot two sons of the names Jnatanika and Sankukarna. He celebrated several sacrifices in the Kuruksetra pilgrimage site, and he had three younger brothers named Srutasena, Ugrasena and Bhimasena II. He invaded Taksala (Ajanta), and he decided to avenge the unlawful curse upon his great father, Maharaja Pariksit. He performed a great sacrifice called Sarpa-yajna, to kill the race of serpents, including the taksaka, which had bitten his father to death. On request from many influential demigods and sages, he had to change his decision to kill the race of snakes, but despite stopping the sacrifice, he satisfied everyone concerned in the sacrifice by rewarding them properly. In the ceremony, Mahamuni Vyasadeva also was present, and he personally narrated the history of the Battle of Kuruksetra before the King. Later on by the order of Vyasadeva, his disciple Vaisampayana narrated before the King the subject matter of Mahabharata. He was much affected by his great father's untimely death and was very anxious to see him again, and he expressed his desire before the great sage Vyasadeva. Vyasadeva also fulfilled his desire. His father was present before him, and he worshiped both his father and Vyasadeva with great respect and pomp. Being fully satisfied, he most munificently gave charities to the brahmanas present at the sacrifice.
Link to this page: https://prabhupadabooks.com/sb/1/16/2