I've been starting to research the subject of menopause as I'm getting to that age where someday I might start experiencing symptoms and want to be prepared.
Basically what is going on is the female reproductive system is slowly shutting down. As a result of this, a woman's estrogen levels will slowly decrease. This causes an imbalance in the body which leads to the list of "symptoms" women experience. How severe the symptoms are, which are experienced, and how long they last depend on genetics, how healthy a diet a person maintains, exercise and consistent rest (good night sleep).
One caution, another side effect of the loss of estrogen is the body doesn't burn calories as effectively, which often leads to weight gain during the process. To counteract this, include strength training with ones exercise program - increasing muscle mass will counter the estrogen decline. More muscle burns more calories. One doesn't have to be a bodybuilder, just a good general strength and toning program (can you do situps, leg lifts, push-ups, etc fairly easily - if not, you need to gain that muscle back...plus you'll feel awesome).
One way you could explain this to him - if he remembers how scary the teenage girls were in middle/high school when their reproductive systems were turning on...this is the same thing in reverse.
Seems like there is more to the story with your brother-in-law, but I find it a good sign he even bothers to ask and find out. Most men think any subject regarding "lady topics" is taboo and should not be touched. If he's really trying to understand what she's going through so he can better support his needs, I applaud his effort. He may not be going about it in the "right" way, but it sounds like he's at least making an attempt. I hope your sister can appreciate that aspect.