I figured it had to do with the fact that it was much more likely that <PERSON> would take in <PERSON> if she felt like she was all he had. If <PERSON> had stuck around, I very much doubt he could have convinced her to take him over her saying simply "you take him, I don't want him". Also, it's possible that she and <PERSON> would have just freaked out if they saw <PERSON>. He clearly didn't have time for that. But I don't doubt for a moment that a watchful eye was always on <PERSON> to make sure he did indeed get in safetly.
To be honest, as awful as she is to <PERSON>, I don't think she ever would have abandoned him. As <PERSON> said, she did take him in after all and didn't immediatly take him to an orphanage. Actually, thinking of it, I would imagine that was the most selflish thing she would have ever done. 86
You can beat your ass <PERSON> didn't want anything to do with him. And why would he, he hates everything he represents (actually I'm curious if he would know anything about magic up to this point. I can't fathom a reason <PERSON> would have ever told him until he demanded answers of where this child came from).
Anyway, we all know how timid <PERSON> is to <PERSON> and how his word is law. I would guess it took some serious guts on <PERSON>'s part and determination to stand up to <PERSON> to tell him they were keeping <PERSON>, which for anyone would be no small feat. Granted, his living situation was awful, but you wouldn't do that for someone if you didn't truely care (now whether it's for <PERSON> or, more likely imo, <PERSON>, I'm not sure).
<PERSON> put his faith in his guess for how much <PERSON> actually cared about her sister and that she generally, deep down, is a good person.
Edit: not sure if my answer got a little bit too of topic. In short, I think <PERSON> had a pretty good guess at how <PERSON> would react. His guess was then obviously verifyed by the events that happened. <PERSON> is, after all, fairly wise ;) 86