Hephaestus and Aphrodite are married, yes?
That technically makes Leo and Piper step-siblings.
Note: I accidentally wrote “half-siblings” but since they are only the children of two married Gods, they would share none of the DNA of the respective god’s spouse, if gods had DNA to spare (which I know they don’t). So I replaced it with ‘step-siblings’ since that’s more accurate. Still. Adorable. ^^