It is probably because there is no address to the sister as the address would be separated by a comma
Praktisch can mean both practically and practical depending on the context. Same with words like definitiv and komplett - they can be used as adverbs
Du bist praktisch meine Schwester. You are virtually my sister. (nominative) Du bist praktisch, meine Schwester. You are practical, (my) sister. (vocative)
What a difference a comma makes.
Yeah, I had the listening exercise, and I thought I heard a pause. Hence, I put a comma between "praktisch" and "meine." xD Luckily for me, Duo still counted it as correct.
I understand the English answer to be synonymous with "You are, to all intents and purposes, my sister" or "You are virtually my sister" (which was not accepted as correct). Is this also true of the original German sentence?
German never discriminates between adjectives and adverbs. That's why Germans learning English often don't know when to put a "-ly" ending to a word. Even "good" and "well" are both "gut" in German.
I don't know if this is enough buy when I hear her say it, she emphasises Schwester, not praktisch. If the sister was practical she woud have emphasised the word praktisch...no?
This seems more like its being used as an adverb, not an adjective in the English.
I started learning German ten years ago, and your comment was the first time I've seen "Bißt" (for what it's worth...)