Sibling problems? Advice needed.?

Ok, so I'm in my late teens and live with my parents who are in their 60s. I'm having problems with my sister who is in her late 20s. She recently moved back to our native city and her roommate (who is allowing her to live for free) says no company, no parties, no nothing. So she solves all her problems by coming to our house everyday from 10 in the morning to 6 o'clock in the evening (no she does not have a job). She pretty much acts like she lives here, using the computer (all day), eating our food (therefore causing more dirt), etc. And at first I really didn't mind until it became a daily habit. But then I said ok she's looking for a job and when she gets one it'll be fine so no need to cause any trouble...But it's been three months still no job AND instead of just her its her and her friends!! These grown women (literally knocking on 30) and they have no where else to go? And God forbid we have our laundry out in our house when she's coming over. She threw a hissy fit. It's like she has no sense of boundaries. And I understand she's lived away for a year and a half but we visited her every 4 months (and she didn't exactly live in the country). Also it's not like she's coming over to be with us cause she missed us...it's because she's bored at home cause they don't have cable or internet and also cause she can't invite her friends over. And she's made that blatantly clear. HOW RUDE! I'm getting to my breaking point. But she doesn't respect anything I say cause I'm younger than she is. My mom is in the early stages of alzeihmers and besides that I think she has selective memory when it comes to my sister anyway. My parents are so wrapped up in trying to be supportive it's like they forgot they still have a daughter at home. I know this is super duper long but one more problem. My sister has been having some problems with her friends so she's been hanging out with mine since shes been here. All my friends are older than me kinda closer to her age and idk I guess I'm kinda insecure about her like stealing my friends cause who'll need me around once they have the older version of me (which I hope I'm not but some people seem to think so). So I got some friends who were my own age. Actually a sibling of a friend of mine's boyfriend haha. And we're super cool the thing is she's graduating next month and she lives a couple of hours away. She invited me to come stay with her and come to her graduation and the party and my sister completely invites herself! I was really mad. I mean it's one thing if you start hanging out with my friends your age that you did not know until three months ago even when I'm not around...but an 18 yr old? goodness. I kinda feel like she's trying to take my place in life or something!! But I really hope I'm never like this when I'm 28 goodness. Please help! ADVICE NEEDED.Sibling problems? Advice needed.?
Tell her how pathetic she is. Get a part time job so you can be out of the house when she is there and save your money so you can move out.