No, I think it's wrong with this specific sentence. Not exactly sure why it is wrong, though...
I think it may be because the sentence is kinda like "You look like your sister looks like" in meaning, so in a way "your sister" becomes a subject of her own clause, with another verb that is not really explicitly written. And therefore "swoja siostra" is like saying that she's her own sister.
Why is "twoja siostra" in the Nominative here? Isn't the subject of the sentence "You"? Or is there a hint to be found in Jellei's comment above: "in a way 'your sister' becomes a subject of her own clause, with another verb that is not really explicitly written."
"jak" in such sentences takes Nominative. I think that is because it could be interpreted as this additional, invisible clause.
You look like your sister. Duolingo marked me incorrect and advised correct response is "You look like your sister". These bugs are becoming more frequent! !!!!