Is it just me or are all step mothers unpleasant? and what should i do with mine?

Okay so here's my dilemma, i have a wonderful dad who married a completely unpleasant women. Basically it all started when i was in 7th grade and my step mom went snooping through my room and read my diary. Now as what a 12 or 13 year old girl who is upset with her step mom for complaining about turning a light switch on too loud and writes in her diary, " i sometimes wish she would go far far away." that doesnt really seem like an intelligent adult would start a war over that, but mine did. she swore i was out to get her. Now granted i felt bad that i hurt her feelings so i did try over the years to be nice and help out around the house, but it was never good enough. My step mom is a cold unhappy person. and its not just me she hates. she picks fights with my aunts, hates all my uncles, and complains about every one. My family has tried everything to counseling to vacation bonding. I have done everything i possibly can too, despite the fact she came at with a knife, spit on me, told me i should kill myself, and slammed me against a mirror when i was thirteen. My uncles and brother think im crazy for even pitying her still and trying to be nice. I need some outside the box opinions and advice, what do i do to make this woman like me? I tolerate her for one reason and one reason alone, my dad and baby sister. My dad is getting older and i dont want him to be alone all his life, and my baby sister who is technically my half sister from my dad and step mom, i love her with all my heart. and as having to be a child that was stuck in the middle of a nasty divorce, i would never want that for her. divorces hurt. Im almost 20 now and i still wish i had had a mother who knew how to love her family. so folks, what do i need to do? failure and giving up really isnt an option. i need to do something to make this unpleasant women back off a bit.