Sharilee, first congratulations on 10 years sober. That is a great achievement.
I have never heard of any discussions about excess alcohol consumption affecting ALS one way or another. One of the most important things he needs to do is keep his weight up. If the beer interferes with his eating, as you say it does, or if he's losing weight, you need to try to figure out how to get better nutrition into him.
Also, his drinking is going to affect his quality of life and health as ALS progresses. If he loses weight, strength and muscle, he will have balance problems, and a load of beer in his belly is not going to help. I'm sure it will make caregiving much more difficult.
But if he's been drinking like this for 16 years, you and I both know he's probably not going to stop when he's hit with a whammy like ALS. (My sobriety date is 1979) Unless he has a transforming experience. Do you think he is capable of cutting back? If so, I'd say DEFINITELY it would help him to get good nutrition and protein into his system. As ALS progresses, he will have less and less control over his intake ... and he may require a feeding tube, which would allow your sil to get some good nutrients in him, too.
If there's any way you could convince him to cut back and replace at least some of the beer calories with more healthful stuff, that will help his overall health and give him a better fighting chance with ALS. And the sooner he cuts back, the better. But, if the beer helps him cope right now, I'd say, just try to get him to taper off so he can take in more healthful foods. Don't take away his crutch completely.
IMO, going cold turkey on booze would be the wrong battle for him to be fighting right now. He's got enough on his plate with ALS. This will be a difficult task for you, but I do suggest trying to help him cut back.
Cindy's advice is excellent ... Alanon would be of enormous help to you and your sil, but frankly I think ALS trumps everything right now. It's up to him if he is willing to fight ALS.