Yeah, there’s a remote possibility of that. Even if it were so, it would not absolve the mother from blame in the case of her illiterate son.
This is where you’re wrong. There is enough information in the OP to determine that the mother is very likely at fault for her son’s inability to read.
Even if one or all of the straw-grasping exculpatory scenarios you have suggested were true, (and that seems like an extremely remote possibility,) the described behaviour of the mother exacerbates and perpetuates the problem.
Look, I spent three years as a (volunteer) one-on-one literacy instructor at an adult learning center. I taught people who were illiterate for a number of reasons. For some, English was a second language. Some were held back by dyslexia or ADHD. Most, however, simply missed out at the beginning somehow, and went through school and life avoiding reading. No matter what the reason for the delay is, these situations are exactly the same in the most fundamental way. Two things are required in order to effect a change: A person who wants to read, and a person who wants to help them to read.
The mother and the daughter were not helping. You know what I heard again and again in those three years? “My wife used read for me, but she died. So here I am.” “My brother helped me out, but he moved out of province. So here I am.”
I will never understand why someone would “cover” for an illiterate person, rather than encouraging them to learn to read. That’s not helping.
Even if momma did adopt an iliterate kid – she has responsibilities. Tutoring at home. Facilitating tutoring or placement in an appropriate class or school. Encouraging the kid to read, instead of enabling his illiteracy. This last is particularly important, and this was demonstrably not happening. When that sort of complacency and apathy is apparent, and it’s clear that the kid’s between seven and ten years behind the program, it’s the safest bet that it’s her damned fault.
Also, the t-shirt choices do underline that she doesn’t really think about her kid’s welfare very deeply. Sending a subliterate kid to school in a t-shirt that says “I heard you, but I still think you’re stupid” is tantamount to child abuse.
Or maybe the kid’s an honour student who just happened to be suffering a Transient Ischemic Attack in the store. WE JUST DON’T KNOW!
The subtle science of duck identification comes into play here, just a bit.