Quote: Would they remember each other?
I believe they would. They parted during a significant, emotional, and traumatic event that must have left some sort of impact. You would have to imagine that this girl carries a huge burden of guilt and shame for her mistake that not only caused the deaths of two people who cared for her but also the pain caused to this boy.
As for the boy, that day had to be the worst thing he had ever experienced. He lost his parents, lost a friend, and was forced to endure years of abuse. He can probably trace back all his problems in life back to that day.
Would they recognize each other is another story. How people look at age 8 and 9 is very different than at 17 and 16. Some people look nothing like their younger selves after puberty, others do. Does she recognize him but he does not? Is she terrified that he'll find out who she really is?
However, as Lycanis said, everyone has idiosyncrasies. Sometimes people grow out of them and sometimes they don't. It could be a specific idiom they use, the way they compose themselves when thinking, how they pick their nose  , sound of their laugh, or how they always wipe their hands after touching a piece of food.
Quote: Would they still think of the other as a sibling or would they just be another stranger? Keep in mind, both of them are loyal and sentimental types.
Eight years is a very long time. Personalities change, tastes, opinions, especially during the time from childhood to adulthood. I've seen people change significantly in only one year, for good and for bad. In my opinion I would think they would be strangers, but it's up to you to decide.
Quote: Would the guy blame the girl for what happened to his family?
If he's cynical he'd probably blame her. Even if he wasn't cynical, from your description it sounds like it was her fault, so why shouldn't he blame her? Whether he has already forgiven her or not after eight years, or if he ever does, is up to you. Time does not heal all wounds. It can become infected and spread throughout his personality, poisoning his mind.
Quote: If she were in a dangerous situation, would he still look out for her and vice versa?
She probably would. Either out of true compassion or as some sort of way to repay him for her mistakes.
The boy is more complicated. He could do nothing out of hatred, do something because it is morally right, he could reluctantly help because it's the knight's code of honor even though he wouldn't otherwise, or ignore the code completely.
To be honest, you have some tough decisions to make. From this setup there are many roads your story can go down and many more ways your characters can turn out to be. My biggest piece of advice is to remember that who your characters are at the start of your story is not necessarily who they have to be at the end.
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. The idea of them not recognizing each other is starting to sound more interesting.
