Spouse inequality & weird parent-daughter arguments?

I live in a pretty calm, nonabusive family with my younger brother, mother, and "godmother." I don't want to ramble about the particulars of my mom and godmother's relationship, but I'd like to retell a short argument that I just had with my mom. Let me try to state the dialogue as verbatim as possible:

I was helping my godmother with some math problems. (They are both going back to college.)
Mom: "I guess you'll help her but neither one of you ever help me."
Godmother: (trying to stay nonargumentative but failing) "No, I offered to help you later this evening."
Mom: "You are such a f***ing liar."
Godmother: "I did! In the Jeep today--"
Mom: (interrupting) "Do you EVER shut up?"
Godmother: "Yes."
Mom: "Can now be that time?"
Me: (I've been silent until this moment...) (to godmother) "You should try talking to HER like that."
Mom: (some sort of exclamation) (angrily) "Go to your room!" (As I walk away, to our cousin...) "Who does she think she is?"

She included a statement about punching my f-ing teeth in, but she would NEVER hurt me that way. We are very close. She is a Southern and Italian mother, so she says things like that as a figure of speech, I guess. Anyway, NOTHING provoked her to be so snappy and rude except possibly jealousy. Now, I'm 17 and what I said was disrespectful. She came straight to my room and asked "What did I ever do to you?" angrily, but I flatly said "I'm sorry." and she hasn't returned. In fact, everyone is acting as if she never raised her voice or sent me to my room.
Anyway, here's my point. They have rarely fought (as in, violently) throughout my life, unless they have been drinking (rare occasion). And they BOTH have faults. However, my mom talks to her like this on a DAILY basis. She thinks I prefer my godmother because I always point out her rudeness.
Have y'all seen same-gender spouse inequality like this, where one person takes degradation from the other? It makes me wonder what kind of person my mom is to treat someone like they're so beneath her. True, they've been together for 18 years, but no amount of time together should make someone forget to account for the feelings of another person!

I hear her scorning and cussing at my godmother right now for always being on the computer, doing homework and not helping her instead. And guess what? I caused this further anger. I tried to make an eye-opening statement but I caused ensuing arguments. The worst part is that my mom isn't physically fighting OR screaming, but she's tirelessly making crude statements. I love my whole family, but... why are people so thoughtless? (Including myself.)

Just some thoughts.

 

They have no idea that I'm bisexual. They've always assumed that I'm straight. But yes, I agree that it isn't normal or justified. I'm not sure how we function so happily with such an odd unfairness. Thanks for your reply!