A couple of scenarios come to mind:
We are all speculating that Ethan is Holly's son, but why couldn't he be her brother?? That would free him up to mingle in both Robin and Lulu's romanaces.
If he does turn out to be Holly's son, he would almost have to be a Scorpio. Holly was supposed to have died in a car accident and kept from Robert afterward by her family ( the Durbins, I believe). Perhaps it was during this couple of years that she gave birth to Robert's son.
Seriously, if Laura could give birth to Nicholas and not mention it to Luke for all those years after she returned, I don't think it would be that difficult for Holly to do a similar thing.