I'm seriously about done.

My little sister is DRIVING ME NUTS. My dad recently got custody of my 2 sisters. One is 12 and one is 8. Since he works 2nd shift I'm stuck babysitting. Now before I go huge into this rant, I do NOT live at home with my dad, just very close. My 8 year old sister has no sense of what is ok and not ok to do. She seems to think if she wants to do it, she can do it. Generally (unless something comes up) I watch her at my dads house Mon, Wed, & Fri. Tues & Thurs at my place. That way my 12 year old sister isn't alone all the time too. Well I have a small place right now. Me and DH were just getting on our feet this is what we have for now cause of our lease. I only keep what we have to have here, that includes how much food we have. So she gets over here today and just starts inviting her friend over for dinner. Won't leave my 2 boys alone, which isn't new. Right now they're sick and have been sleeping a lot. She keeps waking them up. Won't stop playing her recorder she got at school. And to top it all off, starting going threw my cabinets with her little friend. Now most of whats there is basics, food, baby stuff, cleaning stuff, but I have my birth control down there, and pregnancy test for just in case. We moved out to get privacy && to be on our own. Between HAVING to watch my sisters and on top of it not even having privacy in my OWN place I'm just about done. My dad thinks I'm kidding when I say I'm sick of it. "OH well put her in time out, or take her phone and kindle away". That doesn't work anymore. She just does whatever anyway. && Now its to the point its effecting my kids and my place. UGH. I love my sister to death don't get me wrong, but this whole I can do whatever attitude is just getting on my freaking nerves. Her mom gave her whatever so she didn't have to deal with her and I'm not about to do that.

I know it isn't your fault, but it doesn't sound like she has much stability in her life, that's probably why she's so out of control.

WOW if a kid was waking up ANY of my children, let alone TWO sick little twins, I would flip out!!! She's be sitting in a corner the rest of the time. Well okay. Maybe the couch, a corner all day is kind of harsh. But trust me, whatever I do, I would make sure she gets the point about not messing with my kids! But good luck. You say she doesn't care about getting punished, but if you punish her long enough and often enough, she will start to care. Nobody wants to sit staring at a wall doing nothing all the time. She will change her attitude so she won't have to be in trouble.
Or, of course, you could tell your Dad you have enough on your plate with the boys and with how she's acting you can't handle the added stress.