Valmiki Ramayana, Bala Khanda says, that King sagara of Ikshwaku dynasty had two wives. The elder wife of King Sagara is referred as Kesini, daughter of King of Vidarbha . Asamanja, was born to her. Asamanja's son was Anshuman. Asamanja was banished from the kingdom by his father King Sagara, as Asamanja used to indulge in otracious act of throwing little children into waters of sarayu and see them struggle and die.
The younger wife of King Sagara is named Sumati. She is referred as daughter of Sage Kashyapa( Aristanemi).
Valmiki Ramayana further says , this Sumati is verily the sister of Suparna or Garuda, the celestial carrier of Lord Vishnu.
Sumati gave birth to 60000 valiant sons who were later burnt to ashes by the great sage Kapila (Avatara of Lord Vishnu), when they went in search of the sacrificial horse to the lower worlds. Bhagaritha brought ganga for emancipation of these 60000 dead sons of Sumati.
We have heard and many know that Garuda was born to Sage Kashyapa prajapati and Vinata. Another son born to them was Anoora or Arunaa?, the elder brother of Garuda.
So, the question is "Was Sumati the daughter of Vinata?(the divine mother of Garuda)"
Are there any scriptural references which say that Sumati was daughter of Vinata and Kashyapa?
Otherwise, Why is Sumati referred as sister of Garuda in Valmiki Ramayana?