I would not be too surprised if it turned out that Erin was also Patrick's daughter by Melody, but that's a bit more of a stretch.
In some ways this is less of a stretch because Erin was
born over a year before Adrian and Miriam were conceived. Her age is consistent with Melody falling pregnant less than six months after she first met Patrick.
For the timeline to work - leaving magic to one side - Jakob would have to have had sex with Miel, and Patrick would have to have had sex with Jacqui, almost two years after Erin was apparently conceived. That's certainly not impossible, obviously. We have a splash page showing Jacqui in what looks like it
may be Patrick and Miel's place at Christmas 1988 (255), close to when Adrian and Miriam were apparently conceived. And Cloud Echo's memory showed us Miel (apparently) visiting the New York harem recently enough to be greeted by Dani (823) - which would have her still in contact with Jakob long after Adrian was born.
Regardless, my guess is that Jakob engineered whatever happened in order to get someone with Patrick's power on his team.