I will say that, even if things get all happy (or even lovey-dovey) later on, this family is really not good for him to be a part of. Like, even if they all get really close to him later on, that lingering fear of "if I get close to someone and they find out the secret, they'll be murdered" will still be there in the back of his mind. That kind of thing will fuck him up in the long run, and could probably make it difficult for him to form meaningful relationships in the future (as he'd subconsciously be hesitant to get close to people).
Hell, imagine if they didn't manage to get out of this situation like they did. Could you imagine how traumatizing it would be for him to have his closest friend be murdered because she came to check if he was doing okay?
As well as they may end up getting along later on, this is fundamentally a pretty toxic relationship for him.