Which Surah is the sister of the two long ones?

Perhaps by your question, you mean the two long surahs. It is reported in Sunan Abi Dawud and others that Zayd ibn Thabit said to Marwan ibn al-Hakam that the Prophet, peace and blessings be upon him, used to recite in Maghrib the two long surahs, which are: Al-A'raf, and the other is Al-An'am. As for Ibn Abi Mulayka, he said they are: Al-Ma'idah and Al-A'raf. There is no surah mentioned by the name "sister of the two long ones."