I love my sister very much for the sake of our religion which we both follow. If I enter Paradise, if Allaah The Almighty wills, and do not find my beloved sister there, is it possible for me to see her if she is in Hell? If yes, is it possible for me then to supplicate to Allaah The Almighty for her to be forgiven and with me in Paradise?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Love for the sake of Allaah The Almighty is one of the strongest means of security and safety on the Day of Judgment. Indeed it was narrated on the authority of Abu Hurayrah that the Prophet, sallallaahu ‘alayhi wa sallam , said that there are seven types of people to whom Allaah will give the shade of His Throne on the Day of Judgment, when there will be no other shade. These include two persons who love and meet each other and depart from each other for the sake of Allaah. [Al-Bukhari, Muslim and Others]

Thus, if you continue loving your sister because she is righteous and religious, then you will be guaranteed to be given this shade on the Day of Judgment, if He wills. Moreover, if Allaah The Almighty gives anyone the shade of His Throne, then He will not torture them. Therefore, you should remain steadfast in your faith, religiousness and righteousness, as well as your sincere love for the sake of Allaah The Almighty. Also, keep supplicating to Allaah The Almighty to gather you both together under the shade of His Throne and admit you to His bliss.

Allaah The Almighty will allow persons to intercede for others on the Day of Judgment. It was narrated on the authority of Anas that the Prophet, sallallaahu ‘alayhi wa sallam, said that a man may intercede for two or three men and a man may intercede for a man.  [Ibn Khuzaymah] It was also narrated on the authority of Ibn Jid‘aan that the Prophet, sallallaahu ‘alayhi wa sallam, said that more than the number of the tribe of Tameem will enter Paradise through the intercession of a man from his Ummah (nation). [At-Tirmithi, Ahmad, Ibn Khuzaymah]

Allaah Knows best.