。
Grandpa is given to drinking.
Literal
Grandpa [topic-は] alcohol [in-に] is-immersed-[ている].
~にふける (耽る) ('be absorbed in, give oneself over to') takes its object with に. Used for immersion in pleasures, hobbies, addictions — 読書にふける ('be absorbed in reading'), 思い出にふける ('be lost in memories'). The ~ている gives an ongoing/habitual reading: 'is habitually given to drinking.'