As you noted, repetive behaviors and preoccupations are among the characteristics of autism, but these behaviors are also observed in typical children to some degree. So, I don't think that the examples you provided are cause for alarm. If your nephew has typical social and language skills he would not meet the criteria for a diagnosis of autism. Of course, a diagnosis can only be given after a thorough assessment conducted by qualified professionals. If your family has concerns about your nephew's development you should certainly seek an appropriate evaluation.
You are bound to have some concerns if autism is already in the family. But children can have strange interests and not be on the spectrum.
On the autism forum, if you click on the Health Page link at the top right hand corner of the forum page I have posted the diagnostic criteria under DSM IV for autism and have asked parents to post examples of their childs' behaviour that fits the criteria. This will give you examples of the behaviours the professionals are looking for.
If he has typical speech development then that would usually rule out autism, but might mean Aspergers. But those with Aspergers although their speech is considered 'normal' may still have problems processing language, or may have literal interpretation of language. They can sometimes have a strange tone of voice or emphasis the wrong part of words, or they may have an accent.
But as already stated, to get a diagnosis your nephew would have to demonstrate difficulties across all the areas. If that were not the case then he might not get a diagnosis at all, or might have some autistic traits, or could be PDD NOS.
I am not saying that I think he is on the spectrum, just giving examples of how the diagnosis process works.
I would say have a look at the Health Page. Search DSM IV for Aspergers to see what the difference is. Then just observe him and keep an eye on his development and how he appears in relation to his peer group. If there are any concerns then they should be raised through the GP or Paediatrician and a full evaluation should be asked for by professionals who have experience in diagnosing children on the spectrum.