No question that Rox is Quinn's dad. The Ep6 prologue is definitely a flashback to Quinn's childhood. Rox call's her "Q, sweetheart" and tell her to go back to bed. She's definitely a little kid at that point. DPC probably chose not to depict her visually because:
- There probably aren't too many little kid models for use in the porn asset register...
- Depicting a little kid visually in a porn game might be going too far by some people's standards (even if it's totally non-sexual).
But when Buddy calls her a rat and tells her to scram, I'd expect Rox to tell him off (if Buddy was his son), not put a knife to his throat threatening to kill him. Buddy's reaction is definitely fear, it's probably his first time speaking to Quinn like that and didn't expect such a violent reaction from Rox.
If Buddy and Quinn were siblings and that was Buddy's typical attitude to Quinn, he'd have learnt long ago to watch what he says around Rox. So I'm thinking Buddy is just an associate/good friend who took over the business when Rox died.
It does seem like Buddy's now looking after Quinn's fucked up mom though, so there is still the possibility that he and Quinn are siblings, it's just that it's not blatantly obvious (and my list was for the blatantly obvious, not the theoretical).