She went in to try to spoil things for her sister.

Well, that's all she did in the house, IMO.

She seems very bitter and unhappy, and tried to make sure her sister was unhappy too, and succeeded

  Made me larf that did!  

Yes it seemed that all the witch wanted to do was spoil things for her younger, prettier sister.  Tom said Faye is prettier a while back, and Jem heard that LOL!  Let's face it; it's not FAYE'S fault she is prettier and  younger and got all the treats and best things when growing up.  When I was a young child, my family were quite poor (late 60s/early 70s,) and by the time by bro was born when I was 12, (late 70s) we were better off.... and he got EVERYTHING he wanted and was spoilt rotten.  I was a bit pissed off for a while, but never ever once took it out on my brother; it wasn't HIS fault.  

He actually looked up to me massively, and I think this was partly because I had a good relationship with him...  We got on really well - and still do, and my mum explained when I was older (late 20s) that they felt they let me down and wanted to make it up by spoiling him.  By my 20s, I understood and never resented my brother or my parents for that matter.   So if Jem HAS got a bee in her bonnet about Faye being spoiled, she needs to get over herself.  It isn't FAYE'S fault!