Leviticus 21:3
וְלַאֲחֹתוֹ הַבְּתוּלָה הַקְּרוֹבָה אֵלָיו אֲשֶׁר לֹא הָיְתָה לְאִישׁ לָהּ יִטַּמָּא
For his virgin sister who is near to him and who has no husband, for her may he defile himself.
A kohein may defile himself for an unmarried sister who is “near to him?” What does that mean – they talk on the phone a lot? The ibn Ezra explains that it refers to a whole sister – that is, one who shares both parents with the kohein – as opposed to a half-sister.