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JimE2013 said:
http://www.cic.gc.ca/english/visit/eta-start.asp

Click "apply for an eTA." Select "Mexico." Select "no" for the question "Are you a lawful permanent resident of the United States with a valid alien registration card (Green Card)?" Click "next." The result? "Not eTA-eligible"

As for question two of your post, let me search my bookmarks. It's here somewhere.
The reason you are getting 'Not eTA eligible'..... for the link which YOU posted..... is that your sought information on eTAs for Mexican citizens. Until November 30, 2016, Mexicans must apply for TRVs.

Do check the link I had posted earlier..... and you will see what all of us have been saying is correct + you will see the explanation/dates from.CIC below the result
 
The options I selected are in the post I made. The link I used is the link that people must use to apply for their eTAs, so it's the correct one. What are you missing here? I understand that until November 30, Mexicans must apply for visitor visas. I know starting on December 1 Mexicans must apply for eTAs. However, there is something, a bigger issue, that no one has mentioned here: who is to say that all Mexicans who apply for eTAs will be approved? Again, from what I've read, ALL Mexicans are free to apply for eTAs starting on December 1, but only those with U.S. P.R. will be approved.
 
[quote author=JimE2013 lissuenk=topic=456072.msg5505339#msg5505339 date=1476322397]
The options I selected are in the post I made. The link I used is the link that people must use to apply for their eTAs, so it's the correct one. What are you missing here? I understand that until November 30, Mexicans must apply for visitor visas. I know starting on December 1 Mexicans must apply for eTAs. However, there is something, a bigger issue, that no one has mentioned here: who is to say that all Mexicans who apply for eTAs will be approved? Again, from what I've read, ALL Mexicans are free to apply for eTAs starting on December 1, but only those with U.S. P.R. will be approved.
[/quote]
I cannot explain this in any other simpler way. The result from the CIC eTA eligibility link clearly states that Mexicans can apply for eTAs from December 1 (even if they do NOT have US Green Card.... which applies to your brother-in-law).

Approvals/rejections will depend on individual situations.

If you feel there's *a bigger issue* and you have read that only Mexican citizens with US Green Cards can apply for eTAs/will be approved for eTAs from December 1..... then do post those links/source