jats:titleAbstract</jats:title> jats:pWe present Young Supernova Experiment jats:italicgrizy</jats:italic> photometry of SN 2021hpr, the third Type Ia supernova sibling to explode in the Cepheid calibrator galaxy, NGC 3147. Siblings are useful for improving SN-host distance estimates and investigating their contributions toward the SN Ia intrinsic scatter (post-standardization residual scatter in distance estimates). We thus develop a principled Bayesian framework for analyzing SN Ia siblings. At its core is the cosmology-independent relative intrinsic scatter parameter, jats:italicσ</jats:italic> jats:subRel</jats:sub>: the dispersion of siblings distance estimates relative to one another within a galaxy. It quantifies the contribution toward the total intrinsic scatter, jats:italicσ</jats:italic> jats:sub0</jats:sub>, from within-galaxy variations about the siblings’ common properties. It also affects the combined distance uncertainty. We present analytic formulae for computing a jats:italicσ</jats:italic> jats:subRel</jats:sub> posterior from individual siblings distances (estimated using any SN model). Applying a newly trained jats:scBayeSN</jats:sc> model, we fit the light curves of each sibling in NGC 3147 individually, to yield consistent distance estimates. However, the wide jats:italicσ</jats:italic> jats:subRel</jats:sub> posterior means jats:italicσ</jats:italic> jats:subRel</jats:sub> ≈ jats:italicσ</jats:italic> jats:sub0</jats:sub> is not ruled out. We thus combine the distances by marginalizing over jats:italicσ</jats:italic> jats:subRel</jats:sub> with an informative prior: jats:italicσ</jats:italic> jats:subRel</jats:sub> ∼ jats:italicU</jats:italic>(0, jats:italicσ</jats:italic> jats:sub0</jats:sub>). Simultaneously fitting the trio’s light curves improves constraints on distance jats:italicand</jats:italic> each sibling’s individual dust parameters, compared to individual fits. Higher correlation also tightens dust parameter constraints. Therefore, jats:italicσ</jats:italic> jats:subRel</jats:sub> marginalization yields robust estimates of siblings distances for cosmology, as well as dust parameters for sibling–host correlation studies. Incorporating NGC 3147's Cepheid distance yields jats:italicH</jats:italic> jats:sub0</jats:sub> = 78.4 ± 6.5 km sjats:sup−1</jats:sup> Mpcjats:sup−1</jats:sup>. Our work motivates analyses of homogeneous siblings samples, to constrain jats:italicσ</jats:italic> jats:subRel</jats:sub> and its SN-model dependence.</jats:p>