I think you aren't overreacting but it seems the die is cast. Your nephew already has the game, and probably has already played it. I would keep an eye on what content he is playing through and your sister should know what he's gonna come up against in the main quests in terms of themes so she can deal with it.
When I worked for EBGames I used to tell parents who were dead keen to buy San Andreas for their kids (yeah, that's how long ago it was) that in the first half hour of the game you were going to hear every cuss under the sun. That's ignoring everything else (also it was like $70!!). I wasn't trying to dissuade them to buy it, that's the opposite of my job, but I didn't want them coming back and being like "you didn't tell me this when I bought it, now little Johhny is going around calling everyone and this that and the other". I think you have probably introduced an element of caution to your sister, but you can't press the issue. Then it makes you seem like you're being pushy.
Kids can sometimes handle stuff like this, either because they are mature, or because they don't really get it. And sometimes they decide if its something they don't like. My parents a few times let me play M rated games, but more often let me watch M15 movies, even before I was a teenager. I really wanted to play Conker's Bad Fur Day but I didn't really like it because it was "too adult" (HAH!). I much preferred the kid-friendly Banjo-Kazooie.
And it depends on how he plays GTA. Does he go around beating everyone up (and is that any worse than Mario or pro-wrestling games) or is he in it for the cars (which look gorgeous!) and drag racing? Does he "get" the mission dialogue or is it 'mash A, mash A, mash A' until the mission starts and it's drive here under 1 minute?
Overall, parents should research the products their kids are exposed to. You don't often pick a movie at the theatre on a whim, you have seen a trailer. Same with games, watch the trailer first. Or read the back of the box for goodness sake.
SO TLDR version: not overreacting, good to give her a heads up, and she needs to act as a safety net to deal with any issues that arise now he's probably finished the game.
When I worked for EBGames I used to tell parents who were dead keen to buy San Andreas for their kids (yeah, that's how long ago it was) that in the first half hour of the game you were going to hear every cuss under the sun. That's ignoring everything else (also it was like $70!!). I wasn't trying to dissuade them to buy it, that's the opposite of my job, but I didn't want them coming back and being like "you didn't tell me this when I bought it, now little Johhny is going around calling everyone and this that and the other". I think you have probably introduced an element of caution to your sister, but you can't press the issue. Then it makes you seem like you're being pushy.
Kids can sometimes handle stuff like this, either because they are mature, or because they don't really get it. And sometimes they decide if its something they don't like. My parents a few times let me play M rated games, but more often let me watch M15 movies, even before I was a teenager. I really wanted to play Conker's Bad Fur Day but I didn't really like it because it was "too adult" (HAH!). I much preferred the kid-friendly Banjo-Kazooie.
And it depends on how he plays GTA. Does he go around beating everyone up (and is that any worse than Mario or pro-wrestling games) or is he in it for the cars (which look gorgeous!) and drag racing? Does he "get" the mission dialogue or is it 'mash A, mash A, mash A' until the mission starts and it's drive here under 1 minute?
Overall, parents should research the products their kids are exposed to. You don't often pick a movie at the theatre on a whim, you have seen a trailer. Same with games, watch the trailer first. Or read the back of the box for goodness sake.
SO TLDR version: not overreacting, good to give her a heads up, and she needs to act as a safety net to deal with any issues that arise now he's probably finished the game.