I too think you have a lot on your plate to have a fight brewing, but I understand the nagging feeling and wanting to resolve it.
I agree with the previous poster that there are many dates involved in a divorce (my sister is currently going through one) so it could be just that.
But I have also found when there is something to 'hide' either it is dismissed, you are told you are over reacting or anger is simply used to cover it up. Any time I have found these three issues to arise, there seemed to be something worth clarifying. I think it would be worth your time discussing the subject with your sister.
If you love your husband, and there has been some type of 'oversight' on finalizing a divorce, perhaps you could resolve it together. If it is weighing on his shoulders, he might feel a great release from the whole process. I can't imagine there is something so damaging about it that it couldn't be resolved.
I wish you the best and hope for your concerns to be put to rest.