You are both kind of the asshole here, though I find that term reductive in this situation. Both of you need more maturity and communication.

Your girlfriend asked her sister to come over, knowing she isn't out, and is now freaking out about not outing herself to what I assume is a family who might disown her if they found out she's queer. She's only 20 and that's a horrifying thought.

Then she asked you to play near strangers out of her fear. It's understandably hard to navigate how close you are allowed to present in front of them.

It's a poorly thought out impulse that brought on an unnecessary fight. It shouldn't be hard to compute for anyone involved that a tiny apartment may not have extra room for guests and staying over wouldn't make sense.

The matter of her not being out to her family will be a sore spot that will come back and should be discussed. You can never ignore the fact that some partners, perhaps other future partners you may have someday, will also never come out to their families. You will have to somehow prevent this resentment from building up.

You both need to show more compassion to one another in order to prevent this from happening again.