Hi! You have an interesting question there!
The most important thing to remember when interacting with autistic individuals is that we must not have the misconception that autistics are all mentally retarded or insane. This is the common stereotype among the general public. In actual fact, many autistics dislike being given special treatment, and are often social outcasts. We should understand and accept the differences, and not focus on the fact that they have a medical disorder. Just treat them normally.
And of course, we must be sensitive to their needs. Sometimes if they are a little slow in grasping a concept, or completing a task, we must be patient with them, understanding their difficulty.
I'm sure you'll be able to interact with your cousin just fine!
Soon Kian Keong, member of HwaChong project work group