I dont quite remember where the mentioning of the sister being Jun's mom was, but i remember they talk about Yoki's past in chap 3 where he dated just about anyone and everyone
I remember they once talk about how Jun's personality resemble the sister but im not sure, tho Jun is Yoki's child alright. Might read the whole thing again later when im free
It's hinted that Jun is Rio's sister's child. I don't remember the chapter but something was said along the lines of if Yoki didn't have a past of sleeping around then they wouldn't have Jun. And I think they also alluded to Rio's sister not wanting to become a mother because she was free spirited.
Why do I have a feeling that Jun is their adopted child/nephew?
There's no way Father will cheat on Rio