The Secret of Roan Inish (1994)

Fiona (Jeni Courtney) is a young Irish girl with an unusual family history, including a long-missing baby brother. When she goes to live with her grandparents on the west coast of Ireland, Fiona hears stories about her ancestors, tales that involve mythical creatures called selkies who can shift from seal to human form. After Fiona ends up on the small island of Roan Inish, her family's ancestral home, she believes she may have found her little brother living by the sea.