SIS[p] can be used as input for 3-dimensional \sigma^{\mu } p_{\mu } with 4-dimensional Lorentz vector p and is transformed into PauliSigma[Momentum[p]] by FeynCalcInternal.
SIS[p]\bar{\sigma }\cdot \overline{p}
SIS[p] // FCI // StandardForm
(*PauliSigma[Momentum[p]]*)SIS[p, q, r, s]\left(\bar{\sigma }\cdot \overline{p}\right).\left(\bar{\sigma }\cdot \overline{q}\right).\left(\bar{\sigma }\cdot \overline{r}\right).\left(\bar{\sigma }\cdot \overline{s}\right)
SIS[p, q, r, s] // StandardForm
(*SIS[p] . SIS[q] . SIS[r] . SIS[s]*)SIS[q] . (SIS[p] + m) . SIS[q]\left(\bar{\sigma }\cdot \overline{q}\right).\left(\bar{\sigma }\cdot \overline{p}+m\right).\left(\bar{\sigma }\cdot \overline{q}\right)