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The whole family joined forces and harvested the wheat.
Literal
House-all [subject-が] strength [object-を] combine wheat [genitive-の] harvest [object-を] did.
A rural/farming scene. 家中 ('whole house/household') combines 家 (house, family) + 中 (whole, inside) — note the reading じゅう for 中 when it means 'throughout, all of.' 家中 = 'the whole household,' 'everyone in the family.' Contrast with 家中 read いえじゅう, which has the same meaning but a different pronunciation in context. 力を合わせる is a set expression literally 'combine strengths,' meaning 'to join forces,' 'to work together,' 'to pool efforts.' 刈入れ ('harvesting, reaping') is a noun from 刈る ('cut, reap') + 入れる ('put in, bring in') — the harvesting and gathering of crops.