Most wives outlive their husbands. Women live longer than men the same age and tend to marry men who are older than they are. So, if you're a wife, it is more likely you will become a widow than your husband becoming a widower.
Knowing this, how can you prepare for it?
First of all, when widowhood strikes, don't do anything drastic. Do not sell the house or car. Don't decide to move to another town. You have just suffered a very traumatic experience and your system needs time to adjust to it. Take your time.