A little backstory, my gf and her two sisters live together at my gfs house, which my gf owns. The older sister doesn’t pay rent, or pay for groceries, but does use/eat the groceries my gf buys. The younger sister is 14. Even though the older sister agreed to pay rent initially, she hadn’t for months leading up to just recently, where she got a job which doesn’t pay the best, but is full time and I’m over relatively often, and she is gone most days. Her justification for not paying rent is because she doesn’t make much at that job(which seems to me like a silly excuse because something is better than nothing at all). My gf even had paid her medical bill that the older sister had recently gotten, purely because she was stressed out about it, and for some reason didn’t get it on a payment plan or try anything else to handle that by herself.
My gf is extremely giving and pays for absolutely everything, keeps the house as clean as she can, takes the younger sister to school, and helps the older sister out with financial stuff.
Anyway, back to the present issue.
Long story short, my gf and I were putting up wall panels with glue and nails, and it got a little later than we planned, but we had to finish nailing it in that night, otherwise the glue would’ve dried.
My girlfriend’s sister has work in the morning, and understandably was upset that we were hammering at the wall at 11pm, also because her youngest sister has school tomorrow. We tried to explain that the glue was going to dry and that we couldn’t do anything now that it was late because the glue was already applied, and we were nearly finished.
The sister, kept saying why we shouldn’t be doing it, saying that the youngest sister has school tomorrow, even after we are both attempting to explain that we are nearly finished.
Then, my gf snaps and screams at the older sister, which starts a yelling match between the two(I stayed silent), but overall no direct insults were being thrown. Aggressive things were said, obviously, and that continued for a few minutes, and led to them both storming to their rooms.
I walked into the kitchen shortly after we quickly finished and were cleaning up, and overheard the older sister talking to her boyfriend(24), calling my gf a nasty bitch, over and over, and saying she didn’t care. She then went on to talk about how nobody should talk to anyone like that and was using mocking tones to describe what both me and my gf said during the beginning.
I feel mostly split about whether I should tell my gf or not. On one hand, that was a really nasty thing to say about my gf, especially since she’s letting her stay in her house for free, free groceries, and financial assistance(my gf even helped pay for her dog). I would want to know if someone I cared about and was supporting was saying horrible shit about me.
On the other hand, emotions were high, and the fact the older sister didn’t say it to my gfs face counts for something I suppose. If I did tell my gf, things would certainly get much worse.
So that’s my dilemma right now. I was hoping for some other voices on the matter because 1. I don’t really know how girls work, and 2. I don’t want my gf to be shit on and taken advantage of. And I’m autistic(yes diagnosed) so that makes things harder to understand the social aspects.
Thank you all in advance.
TL:DR; Overheard my gf’s sister saying nasty things after an argument. Wondering if I should tell her about it, or if that will unnecessarily escalate the situation.
Edit: my gf is racked with guilt and is crying a lot, saying she wish she didn’t say the things she said(which I feel were totally valid especially with how stressed she’s been recently). I don’t want her to feel like a bad person, and yeah, it was a bit much to start yelling but it was understandable.