For a bit of context, we are twenty years old female fraternal twins. We are really close, though we argue a lot, partly because "she doesn't know how to express her feelings so she becomes agressive, that's how she is" (according to my mother).

I'm going on a trip to Australia for five months in a few weeks. My sister is really angry at me and keeps telling me that I am abandoning her (well both my parents also keep telling me that they're going to be really sad without me). She keeps attacking me and I am tired of it. Today, she yelled at me and called me disgusting and selfish. The reason ? I booked a flight  when she won't be able to come and see me off at the airport. I feel really guilty about that: I knew it would be difficult for her to come, but the flights were less expensive and as my father paid for the plane tickets, I wasn't going to make him pay more than necessary. Furthermore, I couldn't choose the day: I'm invited on the 25th of January, so I have to leave on the 24th. 

Now I'm split between two feelings. On the first hand I feel really guilty for being that selfish and for making her suffer. I care for her a lot (although she says I don't) and it just makes me grieve  to see that I harmed her so much because I decided to put myself first. Then on the other hand I'm really angry at her for insulting me and using her sadness as an excuse for this behavior. I also feel like I have my own life and that sometimes, I should also be allowed to do what is easier/better for me...

So, according to you, Am I the asshole for leaving my twin to go on a six month trip and for putting my own selfish desires first ? 

(If you think my text isn't really clear, feel free to ask questions)