In this sentence, האחיותו של החייל was marked wrong, and it required האחיות של החייל. In a previous sentence, האחות של אמי was marked wrong, and it required האחותה של אמי. Why is the suffix wrong for the first one, but required for the second one?
First of all, correct form would be אחיות החייל. However, construct state would not be used in such a sentence. It doesn't sound natural. In Modern Hebrew, construct state is not usually actively made with new nouns - only set phrases.