Letting children play mature (18+) video games?

So recently I was at my mom's house. My dad and brother were playing grand theft auto and their character was getting blow jobs and sex from a stripper in the car. When my dad told him to stop because they were running out of money my brother says "I'll kill the bitch after and get my money back". I brought it up to my mom and she said that her and my brother have talked and he knows it's just a video game and he isn't to act like that outside of the video game and if he is ever caught mistreating a women she will disown him. He understands but I was flabbergasted that it is even a video game. Who creates such vulgar video games! Will you let your child play mature (18+) video games?