Complex sentence: The baby is a normal human at first, but when the baby gets sick and Lucia tries to bring her to the hospital, it becomes apparent that when the baby's feet get wet with sea water, these change to a mermaid's tail.
Split into simpler sentences: The baby is a normal human at first, but when the baby got sick, Lucia tried to bring the baby in the hospital. Apparently, when the baby's feet got wet by the sea water, it changes to a mermaid's tail.