Translation:As you know, my brother cannot read.
«Откуда» means “where from (whence)” and has two main usages:
- You can ask about where from something/someone is literally going: «Откуда вы идёте?» — “Where are you coming from?”. Its complements are «где» “where” and «куда» “where to, whither”.
- In a more figurative sense, it can be used to ask about the source (of information) or cause: «Откуда тебе это известно?» — ”How do you that?”.
Using «как» would be really a stretch here, it's simply not how it is normally said.
I myself am Russian, and I can say with full confidence that "как ты знаешь?" totally flies as "how do you know?" It's not grammatically correct in any sense, but modern Russians don't really care. This translation may not fly here but any Russian speaker will understand what you mean.
[ How do you know? ] ‧ implies ‧ [ from what reliable source and/or analysis do you present this declaration of fact? / How have you arrived at this posture? ] ‧ Getting, migrating, transitioning from a point of view or opinion to an evolving, developed, evolutionary, revolutionary, or paradigm shift implies motion, which invites adverbs and grammar of motion.
So, how exactly does "kak" work?
First, it function also as a question, so how can you tell the difference between this informative sentence and its equivalent question "Do you know that my brother can't read?"?
Second, in the notes for this lesson we are told that "kak" can function as a "that" for precieved actions. Is that the case here? And if so, why is the "kak" before the "vy znaete" and not before the "moi brat"?