Which Surah is called "the sister of the two long ones"?

The names of the surahs are divinely ordained, and there is no mention of a surah being named "the sister of the two long ones" or two surahs being named "the two long ones."

Perhaps the questioner means by "the two long ones": Al-Baqarah and Al 'Imran, which are named "Al-Zahrawain" as mentioned in the Hadith.

Scholars have divided the surahs of the Qur'an into: the Seven Long Ones, the Mi'in (those with approximately one hundred verses), and the Mufassal (long Mufassal, medium Mufassal, short Mufassal).

This division does not include anything called "the two long ones" or their sister. Perhaps the questioner is referring to Surat Al-A'raf, which was named in the Hadith as "the longer of the two long ones."