Exactly, so just listen to her. Really listen, by looking in her eyes while she talks. Hold her when she cries. Don't try to fix her. Don't defend your BIL. Don't tell how she should feel. Just listen. You might even learn something about how your own W is thinking/feeling.

Men often believe when a woman goes to him to talk about her problem, she expects him to find a solution. No, she just wants to express her thoughts and feelings. That is how she processes and deals with it.

So, just love your sister. Don't judge or preach.
Remember, this is not your W, so don't be telling her stuff that you want to tell your W. Don't let your frustration for W spill over to your sister. Just be her brother.

Try not to take sides, or become involved (apart from listening and being there), b/c you already have a full plate, and you don't need to be in the middle of your sister's battles, if you know what I mean.

It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!