Editing Openai/69186d20-2d14-8013-b9c6-c09000f173f6 (section)

=== Assistant: 2511141758ET ===
251114_1758ET

short answer: Italic (Latin, etc.) and Germanic are the closest large sisters; Celtic + Italic are usually treated as especially close (“Italo-Celtic”), with Germanic next nearest. Slavic/Baltic and Indo-Iranian are clearly further out.

More precise:
# Macro-family: - Celtic is one branch of Indo-European (IE). - All IE branches share a huge inherited core; “closest” means who shares the most ''secondary'' innovations beyond basic IE stuff.
# Italo-Celtic hypothesis (closest sister) - There is a long-running, still quite plausible view that Celtic and Italic (Latin, Oscan, Umbrian, etc.) form a higher-order subgroup Italo-Celtic. - Shared non-trivial innovations (not just “everyone has cases and gender”) include things like: - Similar superlative formation (-amo- / -isamo- types). - Parallel developments in verb morphology, especially future/ subjunctive formations. - Some shared sound changes and lexical items that look too specific to be coincidence or pure areal contact. - Debate is about whether these come from a true common intermediate proto-language (Proto-Italo-Celtic) or from very intense early contact between already distinct Italic and Celtic; but one way or the other, Italic is clearly the closest big neighbour.
# Germanic as next-nearest neighbour - After Italic, Germanic shows a fair number of shared innovations with Celtic that other branches lack (details get technical: specific sound changes, lexical isoglosses, some syntax). - Many historical linguists are comfortable saying something like: > - Again, it’s partly contact, partly genuinely shared developments.
# What you should remember If you want a 3-step mental map: - Celtic is Indo-European. - Within IE, its closest substantial sister cluster is Italic (Latin & friends), often grouped as Italo-Celtic, whether by descent or by intense early convergence. - Just outside that, Germanic is the next strongest associate; Slavic/Baltic/Indo-Iranian sit further away in terms of distinctive shared innovations.

So “closest family” in the sense you care about = Italic, with Germanic as the next-closest big branch.