My love and my sis (14)

1 Name: Secret Admirer : 2008-03-04 15:10 ID:OkUYK83F

Some time back, about half a year, my sis started at a new school.
She lived there during the schooldays, and came here at the weekends.
One day, she called me.
She and her friend, let's just call her Lisa, were bored and tought about having some fun with me.
It didn't really take that long time before this call was almost only between me and Lisa.
I tought she was funny.
Some time went, about half a month, and then my sis once brought a friend over for the weekend.
This was Lisa.
She and I came along well pretty much at once.
It wasn't really anything between us, more like friends.
I started having contact with her via phonecalls, MSN, even WoW sometimes (I actually started playing again just for her sake).
Slowly, I started to fall a little in love with her.
As time went, my sis started to react badly with how I had contact with her friend.
She started to make sure that we didn't have contact.
For instance, she made sure her friend could only come over in weekends she knew I was going to be away on.
Both Lisa and I made it somewhat clear to my sis that she couldn't do that any longer.
But she didn't listen, so we just met behind her back.
Nothing love-related really happened.
She didn't have any problems with either of us, as long as we weren't in the same place.
But that somehow changed.
Lately, shes been really cold against me, often reacting agressively over the smallest thing.
I tought that it just was one of those times again, as she often would be putting up a fight if she didn't get her will, but this time it's been keeping up for quite a while.
And just now, about fifteen minutes ago, I got a call from Lisa.
She was almost in tears.
My sis had said several nasty things to her, how she "could just be with me already", that "she could get out of her life", etc.
And she was devastated, because my sis was a really good friend to her too.
If Lisa was just any woman, I'd butt out and let it cool down... but I like her.
What should I just do to stop my sis from doing this, and not ruining Lisa?
I don't want to do something to end their friendship either, as she would be devastated by that.

Help!

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