But I thought I could still make a request for no M games since my kid is FIVE. And he will naturally want to hang out with the other kids, not the boring adults
If the issue is that the game will be played in an area where you child cannot help but be exposed to it, you have every right to ask that it not be played.
If the issue is that the game will be played in a separate area, but you want your child to be able to go into that area to hang out with his older cousins, you can ask the cousins if, to be kind to your son, they carve out some amount of time to play more appropriate games with him, but you would be out of line to ask them not to play it at all, if this is how they choose to socialize together and their parents allow it.