Is it permissible for a man to combine two sisters in Paradise, by analogy with Uthman's marriage to the Prophet's two daughters in this world?

There is no impediment to combining two sisters in Paradise because it is not a realm of worldly obligations. The word "Qur'an" is a proper noun referring to the revealed word of God, and it has not been pluralized. As for its linguistic meaning, it is a verbal noun (masdar), and verbal nouns are not inherently pluralized.