I'm sorry about your game. That has happened to me many times. For now, you'll have to enter any non-Summoner's Rift match with an expectation for a lower quality game. Riot has made it clear that they don't care about what happens as far as player behavior is concerned in arams, dominion (in the past), , twisted treeline, other featured game modes, or custom matches. Many actions, specifically chat behavior, are still reportable across the board, but reports in those game modes are weighed as significantly less important than those in Normal/Ranked matches. Riot staff have openly come forward about this in the past. Even in Summoner's Rift, though, an action akin to unnecessarily taking a health pack from under a teammate would only cause a player to be punished if he received multiple reports from players in that match. If you were the only one who reported the sivir on your team in that game, I can guarantee you nothing will be done about it.