Cunning is as cunning does
As Professor Xavier's younger sister, and through him Professor Lensharr's... whatever, Raven frequently found herself hounded by other students demanding to know how it was, precisely, that a nice, friendly guy like Charles Xavier could possibly be a Slytherin when scary as fuck Professor Lensharr was Head of Gryffindor. It wasn't that anyone would doubt Professor Lensharr's courage, as such, and he was perfectly even handed in deducting points from all students alike (the git), but his views on muggles were well known, and everybody liked Professor X. It made no sense.
What they didn't understand, not being Charles's sister, was that he was actually a devious cunning bastard. Raven had never seen a more Slytherin-like Slytherin in her entire life. These silly kids failed to appreciate that House selection was not based on political views, but rather on method, on personality. Erik might be severe and terrifying and prejudiced against muggles (but never against muggle-born; he himself was from a muggle family, a curious incongruence that Charles never failed to take advantage of in making a point), and Charles may be cheerful and likeable in his own arrogant, privileged way - he might love muggles and argue for integration while Erik shouted about persecution, and if that was all you looked at, then by all means, Raven supposed she could see where her fellow students were coming from.
But what they missed, what they didn't see... it could fill an encyclopaedia. Erik was brave and straightforward, always barreling through all the hell life put him through in a grim, determined line, following his path through to the end no matter what it cost him, and Raven could identify with that. She herself was a Gryffindor, after all.
Charles, on the other hand - Charles was an expert leglimens, a master of memory charms. He didn't face people head-on, in a fair fight. He twisted people's minds, shaping them to his own ends, and it was all the worse because he was so self-righteous in his moral superiority. His ambitions may be idealistic, but they were still ambitions, nonetheless; people had a tendency to forget that.
Raven loved Charles, but she'd seen what he would do in order to keep a stray, homeless metamorphmagus as his sister, and she never doubted, not for one second, that her brother was a born Slytherin.