Hey Sevendust, welcome and good for you for being proactive on a situation. I, too, have had arthritis since I was a child and have it in my hands. Repetitive motion I think is more prone to cause carpel tunel syndrom and/or tendonitis. My personal feeling is arthritis will come from repetition if you are prone to it (hereditary) and/or have a situation that may cause it to occur in these joints after many years of abuse. I wanted to give you some suggestions (Erin, feel free to say none of this will help) if you're very concerned (not to sound like an old fuddy-duddy, but I hate to see you have long term problems if you can avoid it):
1. Look in to stretches you can to to make sure you don't get carpel tunel syndrome or tendonitis. These are VERY painful and once you have them, you may always have problems. Erin suggested talking with your doctor or school nurse, they might be able to take a look and suggest some stuff. Stretching would include the hand, wrist, arm and shoulders - as carpel tunel pain can migrate all the way up.
2. You might want to try massage. I don't know if it'll help or not with the straightening of your two fingers, but you never know and it may help with the wrist/hand muscles to help keep you from getting hurt. Use lotion and massage and stretch your wrist and fingers. You may want to include your forearm.
3. Learn to type correctly. That way, you are not overdoing the two fingers. I think there are relatively cheap programs you can buy (maybe even free on internet?) that are simple typing programs.
4. (okay, you knew this was coming....I'm a mom....) cut down your time on the computer and/or how you use it. At the VERY LEAST take breaks during your gaming to stretch and relax.
I don't want to scare you nor do I want to lecture
but I have seen a lot of people deal w/these 2 issues and they are not fun. I know at 14 it is hard to think in terms of the future epseically since you don't hurt, but do take care of yourself, do the research on what to do/avoid, stretch, and monitor your time on the computer so you don't have to deal with them in the future. Trust me, you'll be glad you did when your co-workers in 10 years have such painful numbing in their arms and hands that they can't feel them and you feel fine.