I've got a similar situation in my game.
Backstory: I use MC Command Center mod so random townies get married/have kids (not necessarily in that order either). Summer Holiday had twin boys (Emory and Cameron) with Gavin Richards (he had about 9 kids with different Sims before he married Ulrike Faust), then she married Gunther Munch. My Sim (Cassidy Purdue) struck up a romantic relationship with Gunther at the Romance Festival (he's actually kind of cute underneath that dorky hat!) and eventually had his son, Jaxon. I didn't want Summer to automatically be Summer's enemy so I got Gunther and Summer to fight (storyline on that is she never really wanted to have kids and she took out her anger with Gavin on Gunther and the boys) and she divorced him. She sent Cameron (who looked just like his dad Gavin) to live with Gavin and Ulrike (Ulrike wasn't happy with that, Cameron reminding her of Gavin's wandering eye days), and was going to send Emory to Child Services (aka, deleted from the game) but Gunther grabbed him and took him to live with Cassidy and Jaxon. Gunther and Cassidy eventually married after they had a daughter, Michaela.
In relationship panel, Emory is listed as "roommate" to the rest of the family, even to Gunther his step-dad. He had the Clingy trait as a toddler, and now he occasionally gets tense around Jaxon and Michaela because they're not really "family members" (but not with Gunther or Cassidy, his caregivers) so he writes in his journal a lot to relieve the stress. So I'm assuming that eventually there could be romantic interactions between him and the other two children in the household (as well as Cassidy's brother's children, Kiara, Jamal and Jamilah). I'll just have to see where the storyline takes it.
(Epilogue: MCCC ended up marrying Summer to Morgan Fyres -- well, maybe that explains a lot... :D )
