In the literature we generally read that the Koliya clan, a neighbouring clan of the Sakyans, had two capitals - Devadaha and Ramagama. Moreover, the Buddha's mother Mayadevi and foster mother Mahapajapati Gautami were Koliyan ladies from Devadaha. But the following few facts cast doubt on whether Devadaha was actually a Koliyan town and whether the Buddha's maternal relatives were actually Koliyans.
We read that the Sakyans and the Koliyans resided on the opposite sides of the river Rohini and they often quarelled with each other over the water of that river. Now, the Sakyan capital Kapilavatthu is on the west side of the Rohini. In keeping with this, the Koliyan capital Ramagama is on the east side of the river. In contrast, Devadaha is on the west side of the Rohini.
In her Apadana, Gautami says that she was born in Devadaha and her father was Anjana Sakya.
In the opening of the Devadaha Sutta (MN 101), Devadaha is mentioned as a Sakyan town.
These facts strongly point to the possibility that only Ramagama was the capital of the Koliyans. Devadaha was actually a Sakyan town and the Buddha's maternal relatives were also Sakyans. There are few additional points that strengthen this conclusion:
Since the Sakyans and the Koliyans were opponents, it is unlikely that they would make marrital relationship with each other.
We know from Bidudabha's story that the Sakyans were so proud about their nobility that they were unwilling to make relationship even with the mighty king of Kosala, Pasenadi. So how could they make relationship with the small clan Koliya?
The Koliyans, being a small clan like the Sakyans, were unlikely to have two capitals.
Is my conclusion correct? If so, I would like to know how the ideas that the Buddha's maternal relatives were Koliyans from Devadaha entered the literature. Are there any evidence in support of this in the Pali canon or commentaries?