Girls...
I think that Sybill is a witch for Meredith. But she had reasons enough to be like that.
I know that cause I kinda did it once (don't think I'm not ashamed of myself...I'm a mean, mean girl
)with my ex-almost-brother-in-law. My sister was about to marry this guy, but we all, in the family knew that we wasn't the right pearson for her, so, what we did: DISTURB AS MUCH AS WE COULD. That was not a very good thing, but it worked. So, I think Sybill saw her last chance to make her son happy, and TOOK IT(but, yes, she had to be a witch to accomplish that).
Just to close this point i'm trying to make, i think this situation really fits in a song I love, called "cruel to be kind", that says:
"you gotta be
Cruel to be kind, in the right measure
Cruel to be kind, it's a very good sign
Cruel to be kind, means that I love you..."
She did everything for their own good. that's what I think
Hugs, Naná