TINK...tell your sister that she needs to read DR NOW and let him go. I know it is easier said than done...do they have kids?? If not, it should be a little easier to just make him see that he married her such a short time ago and he may not really like being without her. But he needs to see what that is like ASAP. If she just let's him go now she will not push him away any further. I wish i had done this in the beginning, but my sitch is so different. I just htink that if there is any chance he needs to see that she will not beg him to stay.
I hope this helps.