僕は日曜日に働かざるをえない父に同情する。

。

I sympathize with my father, who has to work on Sundays.

Literal

I [topic-は] Sunday [at-に] work-[must-ざるをえない] father [to-に] sympathize.

~ざるをえない ('cannot help but X / have to X,' literally 'cannot not do X') is a literary/formal 'must' construction, stiffer than ~なければならない. Here 働かざるをえない modifies 父 as a relative clause. 同情する ('to sympathize with') takes its target with に.