Could ADHD be the reason my nine year old can not read or tell the difference between 16 and 60?

My nine year old can not read or remember how to spell little words like as and on. She also having major problems with complicated sorting that other children younger then her can do. The school says it is because she is not on medication for her ADHD. She has a younger sister who has ADHD and is not on medication and she is doing well in school. Can ADHD cause all her problems or is there something else going on.

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