I think of all the romcoms where the MCs have a stagnant relationship, no progress over 100 chapters. That's where a love triangle is necessary! This...this one just hurts =_=
For some reason I was thinking that the mother already had those 3 when she met his dad. I didn't think they were actually blood related even though they kept saying dad I thought he was just like a really cool step-dad to them, until he ditched them like he ditched Wahei of course :/