Difference in the inferred SALT2 m_B versus the difference in the inferred x₁ (top) and c (middle) for each sibling pair in the sample. The siblings with large differences in x₁ (and similar values of c) predominantly constrain α precisely, whereas those with a large Δc constrain β. The colour bar shows the x₁ for the wider SN (i.e. higher x₁; top) and c for the redder (i.e. higher c; middle) SN Ia in the pair. For better visualisation, we plot the Δx₁ versus the Δc in the bottom panel.