I also agree that anything on Wikipedia should be taken with a grain of salt, which is why I said "rumored" and "IF it's true."
And I agree with the Alex and Maria idea! They should be depicted as cousins/best friends (though I prefer the former) who live right on the edge of Elwood City limits, and who are mainly friends with the Mighty Mountain/Glenbrook/etc. kids who live near them. That would explain why they are never shown hanging out with the Arthur gang. It's genius!
Now your thinking what I'm thinking.
That could be a good explanation as well as a bunch of other things that have yet to be explained. Also Jenna could potentially get involved. In my opinion I don't think enough was done with Jenna. They could also add other characters seen at Lakewood as well. It would further more backup what you say about not being seen with the gang all the time. I mean Alex and Maria are with the gang in some episodes outside of the school, but not all the time.