Is my sister, who died at the age of 14 after severe suffering from illnesses, intense pain, and surgeries, and then her death was sudden, in Paradise or in Hellfire?

It is part of the creed of Ahlus-Sunnah wal-Jama’ah not to definitively assert that anyone will enter Paradise or Hellfire, except for those about whom a text bears witness to that effect. This is because revelation ceased with the death of the Prophet (peace be upon him), and only Allah knows the true outcome [of a person’s life]. The Muslim who affirms Allah's Oneness will enter Paradise; they may be punished for their sins or forgiven, but their ultimate destination is Paradise.

Muslim children who pass away before puberty will be under the care of Ibrahim and Sarah, and they will intercede for their parents. As for those who reach puberty, they will be recompensed for their deeds: if good, then good; otherwise, it is under the will of Allah.

The affliction that befell the questioner’s sister will, Allah willing, elevate her status, for Allah expiates the sins of a believer through worry and sorrow. And whoever dies from an abdominal disease is a martyr. On the Day of Resurrection, those who were healthy will wish they had suffered affliction when they see the reward for those who endured trials. We hope for her sister, if she died patiently and faithfully, a great reward and an honorable status with Allah.