I think there are two main things you need to look at. One is what are your athletic talents, are you agile but not strong, quick but fragile, and so on and find a martial art that fits your physical talents. Of course at your age you are probably more adaptable than an adult, even if youíre not strong now you might be in a couple of years if youíre flexible now you might become less so if you donít use it.
The other thing is what schools are available to you, how much do they cost, what is the instructor like, what is the general atmosphere, are they traditional or modern, are they into it as a philosophy or sport or just about kicking ass and what do you want from them.
If your cousins are happy with their school youíll probably like it too, Iím no expert but it seems that Kung Fu is a good choice for someone with your body type, after all Bruce Lee was sort of skinny. Just donít sign any long term contracts, then if you donít like it you can switch schools.