So, Junior, as far as I understand - your brother had originally filed I-130 for your mother, your sister and you. All 3 of you filed I-485 (AOS) based on the respective I-130s your brother had filed.
Now, you mentioned -
"She filed our applications, as in, my mother's and both mine and my sister so that we would receive our GCs along with my mother's. For some unknown reason, our petitions were processed first, Mine and my sister's, and they had scheduled our appointment. "
Well, there was nothing "together" about the applications - they are ALL separate applications. Your/your sister's application HAD NOTHING to do with your mom's application.
Also, here's the reason why your/your sister's petition got "processed" sooner - because it would have been denied anyway- both of you weren't eligible do AOS. So, the lady WAS RIGHT - if you guys had attended the interview - USCIS would've denied the AOS, and possibly start deportation proceedings against you and your sister. And it wouldn't be because your mom's GC wasn't approved - like I said, that has NOTHING to do with your/your sisters application.
Right now, both you AND your sister cannot do AOS in the USA because of overstay/illegal presence. You have a chance to leave this country and get your immigrant visa from outside of USA (via consular processing) ONLY if they forgive your overstay since you were under 18. Your sister, however, does not have any chance to see a GC in the near future, unless she marries a USC.