Married to Prince George Kaumualii, son of Kaumualii, King of Kauai. George was five years her senior and a veteran of the War of 1812. Due to his mother's commoner status, George was not in line to follow his father as King or even vassal king of Kauaʻi. George was well educated, having been to New England for an education, however, he fell out of favor with his father. Unhappy with the presence of foreigners in the islands and the influence they held over his father and his country, George gathered sympathizers and planned a rebellion. The decisive battle was fought near Hanapepe. Here the Kauaiian rebels were completely routed. Humehume and Betty were brought to Honolulu, where he died less than two years later. Betty was a widow at age 23. George and Betty had a son in early 1821, but the boy died in February 1823. Mercy Whitney described the burial: "A regular procession of two and two followed the corpse. Going into the fort in which the grave was dug seemed like entering a burying ground, more so than anything I have witnessed since I left America." The fort referred to was Paʻulaʻula o Hipo, a former heiau. The infant was part Hawaiian and part foreign, a veritable conjugation of blood and culture. The only surviving issue of this union was a daughter, born in 1823 prior to their exile. The little girl was named Harriet Kawahinekipi Kaumualiʻi, following her father's surname. Betty and George might have had a second daughter in 1821 that was given to another chiefess because George had no desire for a girl.

Hiram Bingham said of her, "Betty was more fair, of more European feature and slender make than most of her countrywomen at the age of 25 or 30; more taciturn, thoughtful, sedate, and retiring than others of equal rank and intelligence. She had derived some advantages from the instructions of the missionaries, and manifested some concern for her salvation. But her circumstances differed little from those of the wife of a petty chief of the lowest rank."