So..did she know her moms killer all through the movie? *spoiler*

yeah me too, it's a great film, heartbreaking at the end.

i think the film hinted at them actually killing her mum, since they were dragging her off from the stall they were trashing and then the film cut to another scene and never addressed what happened there or specifically to her mum again, but it never explicitly said that was actually the cause of her death.

meanwhile that last scene with yeon hee seeing sang hoon instead of her brother for a moment, that might have been memory or it might have been indeed synbolic; i think it's more likely to be symbolic, since in the original flashback we don't see him give her that look. so it's just a haunting coincidence that she sees him in her brother, since that was a similar situation to when she watched her mother's stall getting trashed as a kid, and she also knew what he did, i'm pretty certain. again it's never explicitly stated that she knew, but she called him a gangster once, so she must have known.

that final scene reminded me of the last bit of the last episode of the wire, if you've seen that you'll know what i mean :)

SPOILERS

If we assume that what we saw in the flashback was real, and that it was indeed Sang-Hoon who got stabbed by her mom before she got taken away (and killed?), I don't think she noticed him since he was wearing a thick jacket and a cap. Maybe at the very end of the movie she makes a connection upon seeing her brother rampaging the food stall.

No, she doesn't know. Neither does the brother. She saw her mom get killed, and the brother sees the scars on the arm of Sang-hoon, but none of them two can make the connection. Even Sang-hoon doesn't know he killed their mother.

That's what make his killing by the brother somehow tragic: while she tried to save Sang-hoon, unconsciously forgiving him, the brother kills him, unconsciously avenging their mom and destroying himself by following Sang-hoon's path.

That last part is the only thing the sister thinks at the end, while superposing Sang-hoon and her brother (not knowing, again, that he killed him, BTW).

That's how I took it. Absurdity of violence.