You're allowed to ask as much as you want, no need to feel like you have to mention that this is your final time! x3

I'm always happy to answer questions, which is why I have this page~
Feel free to keep them coming!

As for the question, if with "blend" you mean that they fuck their parents, no. Not really.
...
Kai diiid do his mom once but that's it.

And as for whether they know about Vivi and Kai, yes!
They both found out their own way. Diana just kind of just pieced it together. Like for example how she could hear constant bumping noises coming from Vivi's room, the same ones she tends to hear when the eevee brought home her latest hookup...except for her coming home alone. Or how Vivi and Kai started to leave for random "overnight stays with friends" during the same weekends and leave within less than an hour from another.
Sebastian on the other hand found it out the old fashioned way: Walking in on them (Which come to think of it is a bit ironic).
Both parents didn't really approve of it, but saw that Vivi and Kai genuinely seemed to be happy with another and so weren't against it enough as to intervene or tell the other. So for a while it was a sort of open secret. Everyone knew, but no one said anything because they weren't aware that the other parent knew as well. That was the case until Vivi brought home an egg one day and she decided that it was time they told them as she didn't want to raise it on a lie or get her friend into trouble. You see, up until that moment, Vivi blamed her pregnancy on an old friend of hers that conveniently moved away just as she started getting visibly pregnant. He was completely unaware of his involvement in this lie, so she didn't want to have to constantly come up with excuses for their relationship and why he isn't checking in on his child or create an even more awkward situation should he ever decide to come visit one day. She knew this wasn't a way to live and would only cause more trouble, so this was the time to finally be honest about Kai and her.
Them spilling the bean was an awkward moment needless to say, but both parents kind of just...sighed and accepted it as they already knew she was lying about the father.
This would have been a bigger issue if it wasn't for the fact that the both of them are part of the eevee species and only half siblings. The idea here being that unlike that of most other pokemon, eevee DNA is incredibly unstable and prone to random mutation. This, alongside its irregular structure, makes harmful recessive genes far far less likely to become dominant. It's also the reason why sometimes eevee children can look a bit different than their parents, which is the case with Mia having a floppy ear, unique tail pattern and darker fur, as some genes may just be overwritten by mutation (Which can of course also be argued to cause the opposite and make complications more likely, but this is fiction and I call the shots so it doesn't x3). Now add on top of that the fact that Vivi and Kai only share 50% of the same DNA thank to each having different mothers, and the likelihood of them causing any harm with their actions is barely any higher than that of it just happening naturally with regular couples of other species. (As long as it's just one generation that is.)
But their species being the exception doesn't change the fact that it's still outlawed in their world and the act is frowned upon societally, whether it causes harm or not, so they gotta keep it hush hush around anyone that isn't in on the secret.

But to get back to the root of the question again for a second:
As I've said in my last response, Vivi did walk in on her parents, and it did leave a lasting memory and desire within her that she's been chasing to experience first hand ever since. Especially how her father acted. So in a way she does have the hots for him. But obviously given the nature of what would be her request then, their relationship and his personality, she's afraid of how he'd react if she tried uh...seducing him.
However, even if she had the opportunity to "try him", she's not sure if she really would.
Vivi is a little bit of a masochist, something that awoke within her when she saw what she saw, but she does still have her limits. And she isn't sure if hers and his align. But if she wants to fulfill the desire she's had for all those years now, she'd have to give up on her limits, leave the safety of a safe word and let him do his thing, otherwise it'd just be a compromised experience that doesn't actually give her what she wants or answer her question of what it would be like to have been in her mothers position at the moment in time. She doesn't know how far he can go and how much she can handle, but would have to put herself in a situation that she can't back out of if it gets too much. Needless to say she's scared of that.