In a comment on my 8/15 “niblings” posting, Aric Olnes reports having 20 niblings (sib’s kids), “27 including spouses”. Now, sib’s kid is a consanguineal relationship — of kinship “by blood” — in both of its parts, sib and kid. Including spouses introduces an affinal relationship — of kinship by marriage — into the mix.
A nibling’s spouse would be, technically, a nibling-in-law, but we don’t customarily treat such a person as an in-law; either they’re no kin at all (instead, in some Americans’ terminology, they’re a connection), or they’re treated as equivalent to a nibling (the way Aric treats them); sib’s-kid’s spouse counts as equivalent to sib’s kid.