my sis has got her boyfriend round

I disagree with some people here

The OP was given the responsibilty by her parents of looking after her younger sisiter. Evidently there are house rules, which the younger sister has broken, so it doesnseem as she is taking advantage of the OP and their parents being away-which is unfair. Its all a matter of trust, and to a degree the younger sister is abusing that trust, so i think the OP has every right to be a little bit annoyed. Hell i would be if i was in charge of my siblings..

Our parents are away in Salzburg for the weekend so I'm in charge.
I'm 20, my sis is 17. She didn't ask our parents if she could have her boyfriend over because she knew they'd say no due to them not being around.
This is her 2nd boyfriend and he seems nicer than her last boyfriend, I have to admit, so that's not a problem.
I said he could come round as long as he was gone by 10 (which he will be cos his dad's coming to fetch him) and as long as they didn't get up to anything (as in sex) while I was in charge. She's only been with him a month and doesn't believe in sleeping with someone before you've been with them 6 months so I don't think this would be an issue at the moment.
I'm downstairs in the front room watching TV and they're upstairs in the lounge watching another channel. I went up to give them chocolate cake after we'd had dinner (God, I sound like someone's mum!!!) and had to open the door, because they'd closed it, and basically found my sis on top of her boyfriend, or as good as. I just jokingly told her to get into a more respectable position before I gave her the cake. I then left the door open on my way out and I'm now sitting in the front room again but in a place where I can see the door, and it's still open. I was just feeling annoyed that they'd shut it because my mum doesn't allow them in my sister's room with the door shut, and I think she'd argue that just cos it's not her room, it doesn't mean she can have the door closed.
And yes, it's just awkward! I went upstairs again to change the cable channel (you have to change it upstairs for it to change downstairs for some reason, and it didn't affect what they were watching cos they're watching terrestrial TV atm) and they weren't full-blown snogging or anything but not exactly just cuddling either. It's just really awkward while I'm in charge to have to act like her mum, especially when every time I've gone in there so far this evening they've been slobbering all over each other.
So yes. I don't know really why I'm posting this. I guess I'm just slightly annoyed because by shutting the door it felt like she was taking the piss, taking advantage of my mum not being here. arghhh.

err, this was two nights ago? so case closed....

Frankly I wouldn't want to be here if they did start going at it, but yes, you are right, given my sis being the way she is that's unlikely to happen.
It's just that my mum's going to be freaked enough in the first place if she ever finds out they were here together without her being here, and I'd rather not have the added problem of them being alone in a room with the door shut. They're already on a separate floor entirely so they don't really need the door closed.
As for why I'm looking after her, as it were, I reiterate that I'm not babysitting her or anything, my parents have been out of the country since Friday morning and will not be back until Monday morning. She is a minor and I am expected to be responsible for her over the course of the weekend. If you want some legal weight, I am in addition named in my parents' wills as my sister's legal guardian if anything was to happen to them before she was 18.

What I object to is my sister deliberately flouting my parents' rules while under my responsibility.

My sister did this to me when our parents went on holiday last summer - she brought her crazy boyfriend round who was actually banned from our house for being a loon and treating her terribly for the previous 2 years!

It ended in a terrible argument in which he threatened to kill me

Younger siblings don't appreciate the pressure older siblings are under to keep the peace when parents are away, which unforunately means sticking to the rules they have in place when they are here.

I'm sure Angelil wasn't trying to be mean or petty or anything towards her sister.

theres a difference if someone has been banned and they comeover cos that is flouting the rules, but to complain cos a sibling 3 yrs younger closes her bedroom door is a little psycho.

theres a difference if someone has been banned and they comeover cos that is flouting the rules, but to complain cos a sibling 3 yrs younger closes her bedroom door is a little psycho.