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Italian and Spanish are part of the western romance subfamily, which is further divided into 3 major and one minor family- the Langues d'si are Spanish, Portuguese and Italian varieties while northern French and Belgian are Langues d'oil and southern French is a langue d'oc. Interestingly Catalan is a transitional phase between the Spanish SI and the Occitan Oc, "si" and "oc" both meaning 'yes' (but si is derived from the latin Sic while oc is from the latin Hoc) as Catala uses Si but shares more grammar and vocabulary in common with Occitan. The minor branch is Arpitan, which uses oue for yes.
The other romance branches of course are Eastern Romance and Sardinian, moribund Dalmatian is a western romance language related to italian rather than Romanian. The eastern varieties all say "da", but romanian linguists claim this is a latin origin word unrelated to the slavic "da" (but both slavic languages and eastern romance use 'da' for 'yes'). The Sardinians are a third isolated group, using 'eja' for yes. Speculated to be a last isolated member of a hypothetical 'southern romance' group which does not exist due to Arabic language expansion.