The pair have been missing since Saturday morning after an apparent attempted rescue in Cove Creek, according to Cleburne County authorities.
Authorities told Channel 7 Chetana Armbya, 26, and Narain Armbia, 33, were swimming in open water on a pontoon boat rented from Shiloh Marina.
Around 11:30 a.m. Saturday the woman began to struggle, according to investigators. The brother then swam to help her, but after reaching her was unable to get back to the boat.
The pair have not been seen since. No bodies have been recovered. Neither sibling was wearing a life jacket.
Pulaski County, Game and Fish, Baxter County, and Mountain Home are assisting in the search.
Because the water is around 90 feet deep, searchers are using two boats from Pulaski County with sonar technology.