。
Mother used to be crazy about tennis.
Literal
Mother [topic-は] once tennis [on-に] absorbed was.
夢中 ('absorbed, engrossed') takes the particle に to mark the thing one is absorbed in — an unusual particle choice for learners used to を or が. Literally 'dream-inside,' the word describes total mental immersion. The past-tense copula だった gives 'used to be [absorbed].'