TW: severe postpartum depression, mention of suicide, possible abuse, etc.

My younger sister has a baby and I believe she’s had some form of severe postpartum depression or postpartum rage going on since she had her baby. Some of the things she’s told me she’s done or does/says on the phone with me makes my stomach turn. When her baby was a few months old she told me that she would get so overwhelmed when she very first brought the baby home that she would scream in the baby’s face, throw things across the room, throw bottles and sling milk across the room, etc. and she just kinda laughed it off in conversation and told me that she thinks all moms probably feel that way but she just “has the balls to say it out loud.” I also have a baby who is just a couple months younger than hers but I personally never experienced postpartum depression at all so I’ve had a hard time finding the line between regular postpartum that will pass over time or someone needing to intervene and maybe take her baby for a while while she gets some help. She has made comments like saying she’s going to “shake the baby so hard she turns it’s brains to scrambled eggs” when the baby has cried too long, says that if the baby doesn’t stop then the baby isn’t gonna have a mom anymore because she’s going to kill herself, shes said she’s going to kill herself over her baby more times than I can count on both hands and feet, says that if someone doesn’t come get her kid that we are all going to find their dead bodies, says that she wishes she could load up in the car and go drive off a bridge, sometimes will just sit the phone down when her baby is crying and I can hear her beating herself in the face and head so bad that she’s given herself black eyes, bruises, whelps, etc. She’s said that she wants to sign her rights away or that someone should just call DCS on her, says she wishes she would have given the baby up for adoption, so many other things I can’t think of right now. Just gut wrenching things that I as a mother could not ever say about my baby. I totally get being overwhelmed and getting frustrated but she just takes it WAY to far. Idk if she actually means all these things or if they are just a cry for help, every time she starts acting like that on the phone with me I just get speechless and have offered on multiple multiple occasions to watch her baby while she checks herself in somewhere, I’ve offered to give her breaks or come help with the baby or have the baby spend the night with me and she always declines with some excuse or hangs up on me so I just don’t know what to do. She’s not like this every day it’s kinda back and forth she does still have good days with her baby but like I said I just feel like she takes her frustration entirely too far. She did go get on medicine a while back to help but she is not consistent with her dr appointments so her medicine gets cut off every other month which makes her mental health worse. She lives alone with the baby and the dad has nothing to do with them, I’m just getting concerned because this is still an issue what should I do?