Our society values human rights and the right to bodily integrity is one of those rights. Circumcision with no medical reason to justify it violates those rights.
It is widely accepted that a parent's duty is to make medical decisoons in the best interest of their child, not what they feel emotionally.
Based on these two points, your sister needs to be strong and logical about this. His actions so far sound like the border on abuse. At the least he is emotionally threatening her and being like a child and having a tantrum.
What she needs to do is remain calm and insist that he provide his reasoning. He needs to explain why the pain, stress, amputaion of sexually important body parts (important to both partners, btw), and disfigurment (at least in some peoples eyes) is worth it. Not just be ranting and yelling. But by giving a carefully reasoned argument that others can follow logically.
She may uncover his insecurity and inability to think logically. She may find he is at a loss and can only react in anger and emotion. She has to hold steady and insist
She can provide references for him to read, if he can't discuss it. But he needs to have the disciplin to read and think his arguments through. This will be good training for him, he will certainly have many more issue to deal with and need to have good reasoning, when raising children, based on my experience.
One good reference, because it is written by doctors, is this one:
A simpler one, much easier to read is this one:
Please support her and help stand firm. It sounds like her husband has not learned to have tough discussions which result in consensus. She may have to play the role of the adult on this one, unfortunately. But her son will thank her for it.
One final thought to keep in mind. Leaving her son intact now is not saying he will never be circumcised. Some would argue that doctors can actually do a much better job circumcising later in life, so she can throw him a bone to help him save some face by getting agreement that when her son is old enough to decide for himself, he can always have it done.
Note: The old argument that it should be done when an infant because of pain is a lie, imho. The only reason that was ever used to justify RC was because most men who have actuallly experienced sex with a foreskin before getting circumcised would kill the SOB that did it after realizing what they had lost. Pain management is much easier as an adult. Understanding why is much easier as an adult. Having it done because you want it done makes it much easier to deal with..