I did a search for her kennel name
and it brought up a thread where another puppy had diabetes and a couple were 12 pounds + and had LP. That's quite a few problems from one seller, especially if you add on the problems both Pixie and Dixie had.
Hopefully someday your heart will heal. You gave that little girl more love than her little body could hold and she knew she was your world in such a short amount of time. I can't imagine the pain you've been through. I hope Pixie is doing ok with out her little sister.