Because Annam preferred sons to daughters, Hiatea's mother concealed her pregnancy and had her raised by firbolgs. When she grew, she proved herself with a series of daring feats, cumulating with a battle with a great monster (a Hydra of Legend or the Tarrasque), presenting it as a trophy to her father. Surprised at her valor and the fact that he had a daughter he hadn't known about, Annam agreed to accept her. From then on, Annam agreed that daughters could be as valuable as sons, something that his later daughters Iallanis and Diancastra were very glad of.
Sources: Complete Divine, Defenders of the Faith, Giantcraft, http://www.wizards.com/rpga/downloads/LG_Deities.zip, Monster Mythology, On Hallowed Ground, Warriors of Heaven, Player's Guide to Faerun