Here's that question

There's no way out for you, this is a classic damned if you do, damned if you don't. The lesser evil of course is to come clean to your girlfriend. Be prepared for the return accusations, it won't be pretty. Your only hope is that this may not be the first time something like this has happened and thus you'll have some credibility...
Good Luck...
-Carlos..

What's the danger? if you stay over the sister might pursue you once more? - that you lose your girl because you've told her, her sister is bad?..
to minimise the hurt to your girl you should considert talking to the other sister - people do stupid things - perhaps this was one of the really dumb ones...
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tell her that you are going out with your girl. and that you aren't going there with her...
tell her to leave you alone or you will have no choice but to tell her sister that she is being a bitch...
tell her this is the only warning, if she doesn't get it - then you simply have to come clean; tell your girlfriend exactly what happened...

If you automatically tell your girl , she may be torn between who to believe and if her sister denies it, chances are she will side with her...So I suggest you go to the source: her sister...Calmly sit down the sis and explain to her that what she did was wrong and that as long as it dos not happen again that you can keep the uncomfortable instance between the two of you...Hopefully she will get the hint and there will no longer be that issue...

Better you tell her first than her sister lets something slip and she thinks that you were hiding it on purpose from her. Though it is a serious accusation, it is better to get it out in the air before it blows up into something much worse...
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And repeatedly, repeatedly state, that she is the one you are going out with, that you have no feelings for her sister, and that you did absolutely nothing to encourage or even promote this behavior from her sister...