Self-restraint, carrying on with normal routines (work, school, friends, etc), avoiding each other, putting on false attitudes of coldness/gruffness towards each other. But as it develops into love, seach of these things wear down over the course of story.
Yes, absolutely, but that's a different story from what I want to tell.
No, never. Nobody can know about their feelings. The brother and sister would be mortified if the other one even knew about them.
Yes, but this is one of the things that happens near the end, thus forcing the optionlock. She doesn't even see them together or anything. She sees the brother essentially avoiding the sister and then a lightbulb goes off in her head and she puts all the brother's behavior into context.
The external complications are that they go to school and NOW live together, so their proximity to each other from all the kinds of things that occur naturally from that ends up being the crucible they must go through.
Then there's things like a third party expressing romantic interest in one of them. This makes one of them jealous, but obviously he/she can't express or admit it or else reveal their feelings for their stepsibling. The object of the third party's affection has to reject him/her without exposing the true reason why.
Things like that.