Why "У моїх сестер" and not "У моїх сестри"?
When you're expressing possession with у, you use the genitive case. Сестер is the genitive plural of сестра, and сестри is the genitive singular (and nominative plural, but the stress is different).
Thanks! I thought it was "сестри" because of the Accusative plural. Well, knowing that the Genitive case is the one used here makes much more sense, дякую :)
Well, to make things even more interesting/confusing. In the plural, the genitive and accusative forms of masculine and feminine animate adjectives and nouns (people and animals) have the same form.