、。
Every time I fall in love, my dad tells me the girl is my half-sister.
Literal
I [subject-が] fall-in-love every-time, partner [of-の] girl [topic-は] I [of-の] half-sister [quotative-だって] father [topic-は] says [explanatory-んだ].
An intriguing sentence with a dramatic family secret. 恋する ('to fall in love') is a verb formed from 恋 ('love'). ~たびに ('every time') combines with the dict-form verb. 異母妹 = 'younger half-sister [from a different mother]' — 異 ('different') + 母 ('mother') + 妹 ('younger sister'), a precise kinship term. だって here is the casual quotative marker (from だと + って), not the 'too/even' だって. The whole thing has a slightly soap-opera feel.