"He is the son of my sister."
Translation:El este fiul surorii mele.
This is not "of my sisters" but "of my sister", it's genitive singular here, genitive plural would be "surorilor mele"
For feminine nouns, the genitive-dative form is always the same as the plural form, and this also applies to adjectives. What shows that it's singular is the definite article:
sora mea - my sister
surorii mele - of/to my sister
surorile mele - my sisters
surorilor mele - of/to my sisters