This is the current book I've been reading.
Summary/Review
After having mother being swept away by a tornado, Boom, a confident boy, must take care of his younger sister Mertyle, while his father locks himself in an attic. Mertyle never goes to school because she's afraid of the world after her mothers disappearance. As usual, Boom goes to the harbor to pick up leftover fish in a bucket, but this time it was no fish. It was a mermaid baby, a merbaby! Boom and Mertyle figure out it grants unlimited wishes, but the merbaby puts a curse on Mertyle for using to many wishes. If Mertyle doesn't return the baby home she with become a mossy mermaid! Boom and Mertyle sail to the island of Mermaids and return the baby, the curse was lifted, and the father realizes that he should come out and care for his children.