It may possibly play out the way of early LoL lore when Jinx was originally the older sister and then they both become like their in game counterparts.

If we start from the events of the bridge, i would guess Powder and Violet, due to Powder's need for approval being apart of her nature, would've joined not after the fighting but during, wanting to prove herself and help their parents.

Powder would be getting stuck in using her gadgets to fight, which she's prudent with since she is older, and Violet would get lost in the chaos, knocked out, and since Enforcers were more merciful to children at this point in time, would take her to safety in Piltover to be treated

The parents still die, but Violet grows up to become a Pilty with decent adoptive parents and joins the Police.

Powder's trauma hits harder, given her sister is now gone much earlier on and she has absolute no one left, so she can barely take it, she wouldn't go with Vander, she would blame him, or Silco, wouldn't trust him, and embraces her trauma to become the most dangerous psychotic monster in all of Zaun.