Research regarding autistic children's sibling relationship quality is mixed, although some literature suggests poorer quality compared to children with other disabilities or who are neurotypical. Little is known about how the clinical characteristics of autistic children relate to parent perceptions of sibling relationship quality. We drew data from a subsample of 119 children on the autism spectrum, ages 10-11 years, from an ongoing longitudinal study. Hierarchical multiple regressions tested the extent to which children's autism symptoms, behavioral difficulties, and communication abilities related to four domains of parent-rated sibling relationship quality. We also examined communication ability as a moderator of the effect of behavioral difficulties on parent-rated sibling relationship quality. More severe autism symptoms were associated with lower levels of conflict and rivalry, whereas higher communication ability was related to more relative status/power, but also conflict. Communication ability moderated the effect of behavioral difficulties such that behavioral difficulties were more closely associated with less warmth/closeness when children had weaker communication skills; behavioral difficulties were not significantly associated with other domains of sibling relationship quality. Findings underscore the importance of considering clinical characteristics and multiple domains of relationship quality to better understand how parents view the relationships between autistic children and their siblings.