Is it wrong that I let my "sister" (a really close female friend) sit on my lap during a party with my girlfriend sitting right next to us? And Is my gf 's reaction wrong? (See comments for details)

I don't know if it's wrong or not but it kind of depends on what your girlfriend had to say about it. If she was mad, I could see why. If she was OK with it then I see no problem with it as long as she is your good friend! Was your girlfriend upset about it? If so, I don't think you should let that happen again.

Next time give your sister your chair. Then she will have a place to sit and your gf would be talking to you.

You screwed the pooch on that one

It is disrespectful to your girlfriend. If you can't see that, then you are not mature enough to be in a relationship.

It's entirely individual.

These kind of actions depends on couples and each gonna be vary. If your girlfriend is fine with it then is fine.  If not, then it's not.  It's called mutual communication and respect in each other's will.

In my relationship, these things are fine to me and my girlfriend. We have group of best friends which me and her both knows them and we live in trust and friendship and we have not such problems.

But since your girlfriend left with your guy friend and you still didn't talked with her after two days! Means she's got something about it! Which you need to follow to makes her comfortable and respect her will from now on and make sure you know her pretty well! It's not that hard! Explore her personality and be open to discussions, make sure to know her and respect the way she thinks if you wanna be with her.

Right or wrong is irrelevant .. If your girlfriend is uncomfortable with that level of physical contact then there are going to be consequences . You can claim there's nothing wrong with it all you like, but she will still react to how it makes her feel .

From reading your responses to the other answers ... The fact that you asked her at the time makes it obvious you have no clue, and shouldn't be in a relationship.

For a non-blood-relative to sit in another person's lap has sexual implications.  For you to let your female pal do so, in front of your girlfriend, was HIGHLY disrespectful (at the least).  And it was likely a challenge from your "gal pal" towards your "girlfriend".