dilemma! what do i do???

my sister is getting married at the end of march. i have been invited but my partner hasnt. im very pissed off. i sent her an e mail saying "keith isnt on the invitation", she said "the invitation is for you". idont really want to go, but i know that i will be used as amunition and will be labelled "the nasty sister" if i dont attend. WTF do i do????
we had a massive fight xmas day and we havnt spoken since. i appologised via email boxing day. she also said in her recent e mail that shes still very upset and hurt with me.my response was "i apologised, what more can i say and do. if your that upset and hurt with me then why have you invited me? do you feel obligated? do you REALLY want me to attend?" she relied by saying " the invitation is for you, please RSVP by the 28th of feb."
ARRRRGH!!!!!!!!!!! Help. what do i do? im trying to weigh things up. we are not close, i wont have a good time, she doesnt bother with me, she forgets my birthdays and doesnt call or send cards, she doesnt visit me,we never go out (at least as friends), and we never chat on the phone for a good chin wag! i dont want to upset mum and dad either. but personally this issue that i have with my so called sister is between her and i isnt it? please someone help!!!!
i thought about caalling her but im 100% sure we will have a fight.
HELP HELP HELP HELP HELP HELP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!