So sorry to hear that your sister is having a rough time with her little one and crying! It's been a few months since you posted, how are they doing? Have the doctors thought about it being colic?
I also went through this. It's so sad when they cry like this. And frustrating when we can't soothe them. I think first you have to make sure nothing is wrong. Like a boo boo, diaper, hungry and all of those basics. Once they are not, some babies do well with oral soothing like a pacifier. Not sure how you feel about that but to get some sleep, I'm all for it. However, then you run into the issue that I did that if the pacifier fell out and baby woke up, they cant get it back in themselves so they cry for THAT. It's a never ending whirlwind of crying sometimes. Do you have a good partner that takes half the duty? And since it has been awhile, how is your baby doing now? Not a baby anymore? :>)))