The good news is that, the fact that your MIL's siblings were diagnosed as HD positive has absolutely no impact on the chances of your husband having HD. Yes, he is at risk simply because your MIL still has a slight chance of getting HD (although it's looking like she may have avoided the curse). The onset of HD can take place at any age although most start showing symptoms in their 40s. I'm not a statistician but I'd think that since she is WAY on the other side of the bell curve, the odds of her developing HD are significantly lower than 50% now. Of course, I didn't do too well at statistics in college. :-)
More good news is that things are changing rapidly now. Many knowledgeable people are now beginning to whisper that they expect a cure withing 5 to 10 years. Treatments are beginning to be developed. We had already had our first child before we found out that my wife had HD and we elected not to have another. But that was before the gene was found and before all of the recent discoveries. I'm pretty sure my decision now would be to go ahead and have another child
The only way to put to bed any doubt in your mind is for your MIL to be tested for HD. These can now be done privately and confidentially and for less than $500 in many clinics across the country.
The important thing that I wanted to communicate was that the only person that you need to consider when determining the "at risk" status of your husband is your mother in law.
Best wishes,
Steve