"He is the son of my sister."
Translation:El este fiul surorii mele.
"..surorii mele" - 'of my sisters'
What is 'mele' here agreeing with? Is it the GD singular form to agree with "of me..", or is it the NA plural form to agree with "sisters"?
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