T.T.
You are definitely not too old to feel that way!! Personally, I think they should know how you feel. Write it all in a letter (try to be non-judgemental, but be sure and get your feelings accross). You can mail it, or not, but if you REALLY want them to know how you feel, send it, you may be surprised at their reaction.
They may not even realize what they are doing. Have you always been the strong, independent one? Sometimes the strongest child (pereceved) is the one who gets the least help as they figure you can take care of yourself. Don't get me wrong, I am not defending them, as I find it VERY insensitive to do this to your child.
I only have two children, and they are young, but I cannot imagine treating one child better than the other. Even if I have a favorite, I would do my best to not show it. I'm sure it gets harder as they get older, but my mom (3 of us 48-40 years old) STILL trys not to favor one over the other even though my sister is dirt poor and my brother is a millionare (literally).