Welcome to Inkscape Community!
As far as I understand, Inkscape is supposed to be able to open AI files. But I'm not clear what you're doing. You use File > Open, choose the AI file that you want, but a JPG is opened instead? I'm not sure how that could happen. And I don't have any AI files, that I could test with.
Could you show me a screenshot of the Open With dialog? Or if you might happen to have a small AI file you could attach, that I could use for testing? Also please tell us each step you take.
Aside from AI, there are some issues when using Inkscape with a Mac. You may already be aware, but here is some info, just in case.
https://inkscape.org/en/learn/faq/#Mac_OS_X_specific_issues It doesn't seem like any of those could apply to this problem though.
I found some info in the manual:
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/File-Import.html However, the info about AI files might be outdated. As you can see, the info given is for 0.48.x. But it does seem to indicate that opening AI files made with version 9 and after should open fairly easily. But for files made before version 9, it sounds like you might have to install Uniconverter (which as indicated further up on that page, might not be easy for Macs).
Oh wait, I just had a thought. Are AI files something like PDF files, in that they can contain both raster and vector content? If that's the case, I wonder if the file you tried to open only had JPG content in the first place?
Last thought. How large is the file you're trying to open? If it's larger than a couple of MB, it might be you just need to wait a little longer for the file to open (especially if you have 4 gb RAM or less).
Gotta log off for passing thunderstorm. But I'll be back in half hour or so
