Complex sentence: The south-Indian manuscripts of the Sanskrit "Skanda Purana" mentions that Devasena and Valli were daughters of the god Vishnu in a previous birth, thus their husband Kartikeya is regarded as the son-in-law of Vishnu.
Split into simpler sentences: The south-Indian manuscripts of the Sanskrit "Skanda Purana" mentions Devasena and Valli as daughters of the god Vishnu in a previous life. Thus, as their husband, Kartikeya is regarded as the son-in-law of Vishnu.