Whether altered white matter microstructural property of autistic people also exists in non-autistic siblings is uncertain. The microstructures of a neural tract may not be consistent throughout the whole track. We assessed 38 cognitive-able autistic males (aged 15.8 ± 4.4 years), 39 non-autistic siblings (16.5 ± 5.7 years), and 78 age- and sex-matched non-autistic comparison people (14.4 ± 5.3 years) using tract-based automatic analysis of diffusion spectrum imaging and threshold-free cluster-weighted method. First, we identified segments within the right frontal aslant tract, frontostriatal tract, and thalamic radiation to precentral areas in both autistic people and non-autistic siblings that differed from those in non-autistic comparison people. Second, segments within bilateral cingulate gyri and callosal fibers connecting superior temporal lobes differed between autistic people and non-autistic comparison people but not between siblings and non-autistic comparison people. Third, segments within the left inferior longitudinal fasciculus and callosal fibers connecting precuneus showed increased generalized fractional anisotropy in non-autistic siblings. Our findings suggest microstructural properties of some potential neural segments that were similar between autistic people and their non-autistic siblings may serve as intermediate phenotypes of autism, facilitating further etiological searching for autism. Meanwhile, increased microstructural properties in unaffected siblings alone might indicate compensatory processes in the light of genetic predisposition for autism.