It's probably that individual theater doing it only. Some theaters will increase the age restriction if they feel the contest is "unsuitable" for younger audiences. Though, it's weird that they'd increase it so high to 21. I've never heard of a theater doing that. Even for movies rated Mature, the usual restriction is 18. If you think it's worth it, file a complaint with the theater because I don't even think they're allowed to increase the age restriction on their own, though I could be wrong of course. If it's rated R, 17+ should be able to get in as long as you have your I.D to prove your age. If your companions are under 17, the 17 year old needs to act as a "guardian" and buy the tickets for the underage people. At least that's how it works at the theaters around here I'm pretty sure. But I also don't live in Pennsylvania so it may be different maybe.
What? 21 to see a rated R movie? That can't be right, I live in PA and you only have to be 17 to get into a rated R movie. But, if your sisters are under 17, (i think) you have to be 21 to get them in.