They're really a great match.
Well, my little bro's a scorpio boy and I'm a cancer girl, and we get on great. Any water signs match up well, or at least get along tolerably.
The only advice I can give from the sibling or friendly perspective is..... let him have his space. You'll want to have your own as well. he may let you in when he feels like it, but if you get "leave me alone" vibes leave him be for a while. He'll come around, for sure. Same advice from him, except you as a Cancer may not give off any advanced warning of alone-time, and it may catch him off guard, but not too badly.
The biggest problem is emotional. If he's happy and you're down, one of you will have to comprimise, and it may be the Scorpio, which can possibly cause an imbalance. I'm a cancer dating a cancer, so we learn to balance moods really well, but with my bro it's different. Scorpio will involve himself obsessively in a loved person's moods, wonder endlessly about how to fix it, but may not show it directly or even end up doing anything at all - which makes them feel useless because they want to care but don't always know how to show it. It'll be up to you to talk about your emotions, especially problems, openly. And if Scorpio cares deeply for a family member or a friend, then be prepared to really listen and give advice. They'll know if you're only half involved in a conversation.
In watching my brother date, I can see how girls may be turned off by that whole "paranoid jealousy" issue. That's just their way of showing that they care. If you're honest and loyal, they'll still show concern, so take it as a sign that they really care. I've seen my bro cry over girls that leave him. It's a harmless trait, really. It just looks a little creepy to most people. A cancer should be able to understand that sort of clingy paranoia, though.
Hope that was insightful in some way! Best of luck.
