The idea that Epiphanius claimed the people of India were descendant from Abraham and Keturah appears to be derived from a conflation of two passages.
In De Fide, 12.5 he writes:
But again, I omit the names of many other mysteries, heresiarches and fomenters of schism whose leaders are called Magusaeans by the Persias but prophets by the Egyptians, and who preside over their shrines and temples. And those Babyloanian magi who are called Gazarenes, sages and enchanters, and the Indians' Evilei so-called, and Brahmans, and the Greek's heirophants and temple custodians, and a throng of Cynics, and the leaders of countless other philosophers.
This is actually a list of mystics of which the Brahmans are one. However, it would easy to misread the passage as equating the magi with the Indians. (The Greek of the passage is difficult to understand, I'm told.)
In De Fide, 7.6, 8.1 Epiphanius writes:
But Abraham's gifts to Keturah's children were wealth - gold, silver, clothing, and whatever Abraham secretly hid in their wallets, the "frankincense, myrrh and gold" ... The children of Abraham by Keturah were cast out by Abraham, and settled in Magodia in Arabia. The same gifts were offered to Christ in Bethlehem by the magi who came from their land and, when they seen the star and come, offered presents and gifts in order to share in the same hope.
Epiphanius has tied the magi of Matthew 2 to the descendants of Abraham through Keturah. However, he has not said they came from India, but rather "Magodia" in Arabia. As near as I can tell, this term does not occur outside of Epiphanius in extant works, and in any case its identity has been lost. However, it is clearly not in India.
In conclusion, Epiphanius does not say the Brahmans descended from Keturah. It was apparently via a misunderstanding of De Fide 12.5, combined with what Epiphanius actually writes in 8.1, that Paulinus got the idea.