Your first example is special. 言うこと is almost an idiom (I'm sure it is except I don't find it in dictionaries) that should be understood as "(one's) words" rather than what it looks like. Thus 弟の言うことを聞いた is "I listened to my brother's words", or in English, even "I listened to my brother". If you said 弟の言ったことを聞いたばかりに～, it'd mean that you are focusing to the specific content he said at that time, rather than using the idiomatic phrase above.
The remaining two are easy when you know Japanese has so-called "relative tense", that is, the time marking in a subordinate clause is usually relative to that of main clause.
誰も言っていることを理解できなかった no one could understand what s/he said (was saying)
誰も言っていたことを理解できなかった no one could understand what s/he had said (been saying)
あかりは自分で描いた絵を見せてくれた Akari showed me the picture she had drawn
あかりは自分で描く絵を見せてくれた Akari showed me the picture she would draw
Of course in English, you can say "Akari showed me the picture she drew" for that context too, but I hope you understand the fact behind: she finished drawing before she showed me.