I am not sure about the off-screen talk between Dora and Sven. Dora does have a habit of making rash, irrational decisions in anger - harsh ones, too. No matter how incomplete her information is. Remember when she 'broke up' with Marten when she saw him hugging Faye? What she does in her own time and with her boyfriend is nobody's business, but firing somebody because of that? (Yes, I know, Faye talked some sense into her, but 'Don't come into work tomorrow' is pretty unambiguous).
That was not one of Dora's finest moments. I've gone on record here saying that the very item you brought up nearly had me swear her off for good. There was no excuse for that sort of overreaction.
Still, I just can't see her doing something permanent with a family member based on third-hand info. *shrugs* That's just my opinion. Like, I can't see her having reacted that way to Faye or Marten if someone had just told her that they saw them hugging rather than her seeing it herself.
At any rate, I have a feeling we'll find out soon enough what actually happened there - if anything at all.
Disclaimer: I'm neither pro-Sven nor pro-Dora, so I don't have quite the same emotional investment that some of you seem to have in defending one or the other of them. They're both capable of being nice, even loving people. They're also -- as they've both lately shown repeatedly -- capable of being colossal assholes.
Thing is, there's a lot of backstory we don't know with the two of them. Sure, it's been brought up in small bits and pieces, but I haven't seen anything that'd explain both of them having all the emotional depth of, say, your average puddle. Dora does have resentment that Sven was the golden child and never had to work as hard at things as she obviously has. Sven, who has only one real friend that we've seen in the strip (Wil), likely envies Dora's friendships, which are not only more numerous, but also have a staying power that his own relationships lack.
But you know something? We're talking about two people in their late twenties. At some point, you grow the hell up and realize that you either take the reins of your life or you run the risk of letting your past dictate your decisions for you. Right now, they both fall in that camp, with Sven deciding that it's easier to revert to type than to change, and Dora deciding that she'd sooner cut her brother out of her life than to actually confront him about the things he does and what they say about who he is (she's complained and threatened plenty, but that's not the same as actually having an adult conversation with someone). If their issues are with each other, they need to work that out; if it's with something one parent or the other did, same thing. But in either case, they're both adults and it's about damn time they acted like it.