PauliXi[I] represents a two-component Pauli spinor \xi, while PauliXi[-I] stands for \xi^{\dagger }.
PauliXi[I]\xi
PauliXi[-I]\xi ^{\dagger }
PauliXi[-I] . SIS[p] . PauliEta[I]
% // ComplexConjugate
\xi ^{\dagger }.\left(\bar{\sigma }\cdot \overline{p}\right).\eta
\eta ^{\dagger }.\left(\bar{\sigma }\cdot \overline{p}\right).\xi