Wait, Aveline doesn't have a flaw.... WTF?
Aveline is the most corrupt person in DA2. She's a good person, but corrupt as all hell. She turns a blind eye because she cares about the people breaking the laws she's sworn to uphold as Guard Captain.
I've lost count of many times Hawke broke laws in front of her. Or Isabela (thief and formerly a raider). Or Varric (basically runs organized crime in Kirkwall with Hawke). Or Fenris (kills Tevinters and takes their stuff willy nilly). Or Anders (an apostate abomination). Or Merrill (a maleficar). Or Sebastian (contracted Hawke for several assassinations). Don't get me wrong. These are all characters that I enjoy to varying degrees, but it's time to call a spade a spade folks.
Her direct subordinates were raping elves in the Alienage and she literally turned a blind eye until the Arishok got involved. Guards taking advantage of elves had been going on since Act I and in all that time she did nothing because she didn't want to get her 'family' into trouble.
That's a pretty glaring flaw if you ask me.
I think you misunderstand how the world works. First of all Aveline at least sees her job as more of keeping the peace, she mentions this to Fenris in one of the idle conversations, guards will lt people lie if it keeps the peace. If you're not abducting or attacking innocents she has bigger things to worry about.
Killing people that attack you, is not a crime. Even walking into thier base is probably acceptable. Isabela doesn't actually do that much in the way of blatantly illegal activities, yes she WAS a smuggler. She can't really do that anymore. Aveline could probably haul her in for illegal entry into Kirkwall, ut it wouldn't stop the crime other people are doing. Varric works with the merchant's guild, and while he doesn't pay by thier rules, that's not Aveline's jurisdiction. She's not thier seargent-at-arms. I could go on, but the point is this, Kirkwall has gangs wandering the streets, abducting people for slavers, killing them, robbing them, etc. Aveline is going to worry about that rather than people who aren't hurting people but are breaking the law.