Geeze, was getting ready to tell about our seizure info and then saw what happened. So, so sorry! I've put down 3 pets in the last 2 years and while it didn't make it any easier, they were 13, 16 and 18 - not young like yours.
I suspect that it seems like there was an underlying condition and shit just hit the fan that had nothing to do with the meds.
For the future, I used these guys a few years ago - I realize you can't drive to ATL every time you have a major issue but they might be able to help over the phone:
http://www.gvsvet.com/
Warchild - your sister rocks.
Sorry for your loss - I don't trust people who don't like pets!