。
Sometimes I do the dishes for mother.
Literal
Mother [for-のために] sometimes dish-washing [obj-を] do (polite).
皿洗い ('dish-washing') is a verb-noun compound from 皿 + 洗う, a productive pattern for naming household tasks. ~のために marks the beneficiary — the speaker is doing the work on mother's behalf. ときどき ('sometimes') as a frequency adverb stands freely without a particle.