Judging from a sentence search on “No sabía que”, it looks optional whether to use the subjunctive or indicative in the subclause. I cannot find a consistent pattern to it.
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I’ve found an explanation that makes sense. Apparently, the subjunctive shows that the speaker is doubtful about the other person really having a sister.
https://www.jtspanish.com/2018/06/24/no-sabia-que-indicativosubjuntivo/
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