It's revealed in the original game when you encounter Hojo in Midgar that he is Sephiroth's father. Lucrecia was artificially inseminated with his child as well as Jenova cells.
Vincent was in love with Lucrecia but yeah, Hojo got in there first.

Ifalna and Professor Gast are Aerith's parents. Lucrecia resembles Aerith because they ran out of ideas for character models lol.