For me, if we pursue this sub-plot, Sebastian's character arc is going to feature these main things;

- Suspecting that he is, then finding out whether he is responsible for the potion (let's say as OoC, probably indirectly)
- Internal conflict over responsibility for events and the ethics of his practices
- External conflict over [i]covering his arse and not getting lynched by the witching community[/i]

As for his relationship with Matthew, there are quite a few possible avenues to take this down without really affecting what you'd already laid down. Sure Matthew might have inheritance and money, but he no longer has a family around him. If nobody sweeps in and helps him, there's a decent chance another family or a hunter will pick him off. Apart from anything else, there's three corpses, including that of a hunter, lying on the floor of where he lives. He's going to need some help shifting them because if the police or another hunter spots them, he's in for a bumpy ride. Besides, if he hasn't emerged until now, it stands to reason that he is unpractised in the magical arts and could probably do with some kind of tuition. Certainly as a sort-of member of the Anderson family and an individual, Sebastian can help with those two things - if not directly, he'll certainly have some favours he can pull in.

Of course, their relationship need not be entirely mutual. Matthew may reject help of any kind, or Sebastian may resent having to get so up-close-and-personal with one of his charges. I think it's likely that the Anderson family would put some external pressure on the two of them to establish some closer kind of relationship. Of course, Sebastian can definitely benefit from any arrangement they set up; allies is allies, and when they come with money and/or power and/or a malleable or submissive nature (though I can't find a personality field to speculate about in your character sheet, oddly), all the better. Besides, what can massage a guilty conscience like taking an orphan under your wing?