。
Fish and cockroaches are the only things she's interested in.
Literal
She [topic-は] fish and cockroach [only-にだけ] interest [subject-が] exists.
An eccentric line, structurally tidy. と links two nouns into a single conjunctive group ('fish and cockroaches'), and にだけ stacks the destination/target particle に with the limiter だけ — 'only to' that combined group does her interest extend. The predicate 興味がある is the standard way to say 'be interested in,' with the topic of interest taking に. Cockroaches (ゴキブリ) occupy a particular cultural niche in Japan: warm humid summers make them a recurring household pest in older buildings and especially in Tokyo and Osaka, and they're a stock gag in manga and TV — a screaming character clutching a kitchen sandal. Pairing them with fish makes the sentence read as deliberately offbeat.