It's never made particularly clear what happened between Zeus and Helena, but Helena seems to be aware of who she slept with and bear Zeus no ill will. Regardless, Diana certainly has as much claim to spiritual and adoptive maternity over Cassie as Ares has to Hippolyta. She's never been explicitly declared Diana's heir by the amazons, but pre-Johns it was always still clear that she was all the same.
Now, as we've learned lately, in the DCU Blood is All, so obviously Zeus' paternity takes precedence over everything and any importance Diana might theoretically have once had to an empty ungrateful bile-filled cipher who can't even be assed to wear a pair of goddamn bracers anyway. So I guess it's all fairly academic.