I'm so sorry you're being treated like this. Many warm safe hugs going your way (but not too warm, cause it so dang hot out...). I'm sorry I don't have much advice to offer. I was just wondering if maybe she has postpartum depression... You mention that it's a new baby, and anger can be a result of depression turned outward. I don't have any experience with postpartum so I could be way off here. Could you talk to your dad/the baby's dad to see if he can encourage her to get help or see a doctor? The reason I don't really recommend you trying to do it yourself is because that can cause a backlash of "What do you know? You're just a kid! I don't need help!"

Anyways, just wanted to send you safe thoughts and support your way. I hope things improve for you soon!