could your sisters daughter be a little bit post natal? it might explain some of her anger, maybe she is also feeling a little overwhelmed as well, and maybe if your sister is trying to do too much for her it is crowding her and not giving her a chance to find her own way of being a mum? just a few ideas to suggest to your sis.

I would say the same as the others. Back off. Let her daughter be a mother. Tell her to let her daughter know that she will be there for her if she needs it, but otherwise let her go. She is an adult. Unless the baby is in danger then I would leave her be.

Your niece is probably stressed and angry about the situation. She needs to learn how to relax.

I know you want to protect your sister, but I'm sure she knows what she is doing. Besides, what kind of mother would she be if she decided to leave her daughter completely alone when her daughter is pregnant?

i know what its like for people to "butt in" or "help" when you have your first child as a young adult/ teenager.
From experience i can honestly say that i don't think her daughter is yelling out of disrespect, but purely out of frustration.
It does get quite annoying when people step in and "interfere" with the way you are trying to raise your child, and it makes the new mum feel incapable....
All i can suggest is that your sister tones down abit with the help/ interference or whatever it is.... her daughter will ask for help if she needs it!
Cheers and hope this has helped