Praise be to Allah.
It is part of the belief of Ahl al-Sunnah wa’l-Jamaa’ah that we cannot state definitively that any of the people of the Qiblah (i.e., Muslims) will go to Paradise or to Hell, except for those concerning whom Allaah or His Messenger have testified, because testifying to that effect concerning any person is something that can only be known via wahy (revelation), and revelation came to an end with the death of the Prophet (peace and blessings of Allaah be upon him). So no one knows in what state people will die, or what is hidden in their hearts except Allaah Who created them. But the Muslim who believes in Tawheed will inevitably enter Paradise if he bears witness to the Shahaadatayn and fulfils the duties they require. Allaah may punish him if he commits some sins, or He may forgive him, but his ultimate destiny will be Paradise. We have to believe in Allaah’s promise which gives these glad tidings.
It was reported in the texts that if any Muslim child dies before reaching the age of puberty, he will be under the care of the Prophet of Allaah Ibraaheem (peace be upon him) and his wife Saarah, and he will intercede for his parents on the Day of Resurrection.
But the one who lives beyond the age of puberty will be requited for his deeds; if they were good then he will be rewarded, otherwise he will be subject to the will of Allaah: if He wills He will punish him and if He wills He will forgive him.
What the questioner mentioned, of the suffering endured by her sister – may Allaah have mercy on her – perhaps Allaah will raise her status with Him because of that. For no distress or grief befalls a believer but Allaah expiates some of his sins thereby. It was reported in the ahaadeeth that whoever dies because of a stomach disease (al-mabtoon) is a shaheed (martyr), and on the Day of Resurrection, when those who lived an easy life see the reward given to those who suffered during their lives, they will wish that their skin had been cut with scissors.
Hence we hope that if your sister died bearing her affliction with patience, believing in Allaah, doing whatever she could of the obligatory duties, that she will have a great reward and a high status with Allaah. We ask Allaah to have mercy on us and on her, for He is the All-Hearing, Ever-Responsive. And Allaah knows best.