Mastercontroller doesn't know about woohooer actually so if you're trying rule breaking there you're more likely to encounter resistance. Erasing the relationship and going about it backwards might work but I'd try to make it happen organically through the game with near relations enabled in woohooer to see if your mileage improves.

The game could be seeing both women as the parents of child 2, since there isn't a step parent or adoptive parent setting.