dear Vrmmonter, thanks for coming to the forum.
What I can only say is that your brother in law has definitely got OCD, as hoarding is a typical trait of this illness.
What they do is just pick up anything they like or what they believe would be useful but never use, any type of rubbish, junk and useless material, which no one else would ever need or want to be used, and it's only going to get worse.
He wouldn't think that there is a problem and certainly doesn't need help, so what you or your wife have to say to him will go in one ear and out the other.
Now that your mother in law is getting old and really by her still living at home is a great effort, but now her eyesight is deteriorating could mean that she may have to find other accommodation with 24/7 help, and what is to happen with the house.
Does your wife have power of attorney, and if so then she could then sell the house, when he removes it all.
This maybe difficult to achieve so you may have to get workers in to clean it all up and then tell your b/law to vacate.
Another way to get rid of the junk is to approach your council and get them to put a notice on him to remove all the junk, unfortunately this won't get him to remove it, so just keep going back to your council until something is done.
Your actions have to be persistent, and I know that he should get some counselling, but I think that the rubbish/junk has to be removed first.
Just a quick point and that's I have had OCD for 55 years.
I would really like to hear back from you. Geoff.