It's not. "my daughter" is the subject of the sentence, so it's Nominative.
If you ask that because you automatically associate possession with Genitive, it's not that everything connected with possession takes Genitive. It's actually more or less like in English, where you have Saxon Genitive. For example "my daughter's dog" has this 's part (Saxon Genitive), while "my dog" does not.
In Polish as well, simple possessive pronoun doesn't take Genitive (moja córka), but if some noun is the 'possessor', then it takes Genitive (pies mojej córki = my daughter's dog -> 'my daughter' takes Genitive).