love at once

Shouji has a secret that his mother is a mermaid. It's true. He met mermaids when he was a kid. One day, his father passes away.... Two months later, twin girls visit his house. Their names are May and Meru. Yes, they are actually mermaids he met before. Since his father died, he will be able to fall in love only once. Shouji is totally confused.... May and Meru say to him, "We'll support you, so enjoy your love...."