I think it's pretty much what we'd expect of Sven at this point, though. Whatever else can be said about his character, he's always been shown to care about his little sister - behind the jokes about how she'd kick his arse if she saw a need, it's pretty obvious that he does genuinely care about what she thinks. Which follows, naturally, that he genuinely cares about her. So if anything's going to cut through his suave-womanizer-bullshit, it's the thought of Dora hurting. There's just that instinctive Big Brother reflex that I don't think even Sven is immune to. His feelings toward Faye, whatever they may be, won't trump that. Which is why I think he's being completely honest to Marten, who he respects as a decent guy for his sister. Hell, he basically said as much in panel 2.
But for the same reason I can't see him giving Marten a big history on arseholes from Dora's life. I think he'll basically give Marten the advice that's needed: that Dora, herself, needs to be able to tell Marten about what she's been through, and why that means she has so much trouble trusting him. Part of it is her self-image, part of it is instinctive reaction to a guy she has feelings for, but all of it is coming from her, and that can't be solved by a third party filling in the blanks. She has to be the one to talk Marten through this, if only so that they can come to the conclusion that he's just not That Guy.
As for Sven? I wholeheartedly endorse him giving Marten a nudge in the right direction. It's such a Big-Brotherly thing to do in order to help Dora. But at the same time I don't see him wanting to get any more involved than that, or telling Marten too much. I definitely don't see him wanting Dora to know that he's helping. Like he said himself, "Gotta keep up the act."
I think this is one of Sven's most important appearances since he and Faye had their falling out. It makes him a real person again, it's so believable for what siblings who care about each other would do, and yet it's perfectly in character for what we've seen - he DOES care about Dora, he just doesn't want to admit it to anyone. I'm really enjoying where it's gone for him.