To be honest, I agree that it depends on the game, like those with romance and sensitive influence. Hoever, I love playing games with people around, friends, family or otherwise because of the comentary I hear is amazingly funny. Heres an example:
I was playing Fatal Frame with a couple friends in the room, and not only did they freak out half the time, they also pointed out stuff I missed. Items, rooms, and the like helped improve the experience for all of us, especially when we ran into stuff I havn't seen before.
At the same time I can understand playing by yourself, when it comes to games where you do not want commentary or help. If it's your favorite series or just a continuation to a very charming game, you might feel you want to experience it yourself before letting others in. That much is natural, but you might have more of an issue with your sister and her friends than you do with gaming alone, if they are chatty or obnoxious it might ruin the experience for you.