<p>Recent research suggests that actual and perceived mindsets of socializers can impact students’ academic-related outcomes, including motivation, self-reported behavior, and emotion. In the present study, we focus on a less-studied socializer, <i>older siblings</i>, exploring patterns in students’ perceptions of their siblings’ growth and fixed mindsets and how these perceptions relate to their outcomes in math learning. Using a latent profile analysis approach with undergraduate students (<i>N</i> = 508), we find evidence for three distinct profiles of perceived sibling mindset: <i>high growth mindset</i>, <i>low growth mindset</i>, and <i>ambivalent</i>. When students perceived their siblings to have a high growth mindset, they had more adaptive motivation, self-reported behaviors, and emotions, compared to when they perceived their siblings to have a low growth mindset; those in the ambivalent group had more mixed outcomes. Our findings suggest that the presence of growth mindset messages from an older sibling may help to support students’ motivation, self-reported behaviors, and positive emotions, whereas a lack of growth mindset messages from an older sibling may dampen these outcomes. Implications for theory and future research are discussed.</p>