The same reason it is not seen now by those who have not experienced this. You have experienced death before your husband's, Jo, and from what I've been through and what I've seen, I think you'll agree, no death experience can prepare you for the death of your spouse or for the loss of your child. Of those I've met who have experienced one of these deaths, each prays they do not find out what the other is like.
There are many types of ambivalent or ambiguous grief and what your sister may experience is one of them. Remember though, we're all different and it would be wrong of you to impose or project how you grieve onto her. What she is showing may not be denial and you wouldn't want her to think something is wrong with her because she isn't feeling what you expect her to feel. However like you, if she needs and wants support for her grief manifestations, I hope she is able to find it.
This journey already has dark and stormy skies. I don't know how I'd handle an additional cloud. I hope things will not be as bleak as you fear for your BIL, his children, his new partner, or your sister.
