Practical is an adjective. "You're practical my sister" isn't a grammatically correct sentence. If we were to use it as an adjective, it would be "You're my practical sister" in English and "Du bist meine praktische Schwester" in German. Adjectival phrases are built as "determiner + adjective + noun" in both languages. So, you can assume it's an adverb if you saw it before a determiner like mein, ein, der, dies in German and my, an, the, this in English.