What's the manga where a boy confesses his love to a girl but she turns him down Then when he gets home he finds her and her mother and finds out his dad is marrying the mom making them siblings?

The manga you are referring to is "Koi to Uso" or "Love and Lies" in English. It follows the story of a boy who confesses to a girl he likes and later discovers that their parents are getting married, making them stepsiblings.