Nor am I claiming he's a saint. He's been an incredible asshat. The difference is, he owns up to it without the slightest attempt to justify or defend it. And he does indeed care about his sister. The examples I gave, the only ones given of him actually lying... they're lies for self-preservation, not self-gain. He doesn't want to get hurt by his ex, he doesn't want to be tempted by a night of sex with an old flame, and he doesn't want Faye to be upset and angry with him. He's not trying to get anything out of these lies except to avoid an unpleasant situation.
Maybe I'm only splitting hairs with the difference here, but I think there is one. He doesn't walk up to a lady and make her impossible promises in order to get laid, he doesn't lead women on and trick them, and he doesn't just tell women what they want to hear to make himself look good. On the contrary, he seems to go out of his way to make sure they know what kind of fellow he is and that there are no illusions between them of something significant when he thinks it's just about sex.
He uses women, and he is indeed unchivalrous, but whatever his other faults, he's honest. And I think, especially when it comes to his little sister, he'd maintain that honesty. The only problem is, as I first said in full agreement with others, is that his perspective of Dora's issues would not take in the full picture. Not to mention that since it's about (amongst other things) his little sister's sex life, he couldn't really be of much use if confronted with a full perspective.
Sven has done some things that make him a bit of an asshole, but he's not the devil incarnate. He has feelings, and emotions, and a conscience, and he's really pretty open about most things. That just doesn't mean he's a good choice for Dora to talk to about her issues, is all.