Praise be to Allah.
and blessings and peace be upon the Messenger of Allaah.
We ask Allaah to have mercy on her, and to join you with her in the Gardens of Delight (Paradise).
The Prophet (peace and blessings of Allaah be upon him) said: “Every child is born in state of fitrah (the natural disposition of man), then his parents make him a Jew or a Christian or a Magian.” (Narrated by al-Bukhaari, 1296; Muslim, 4803).
And he said: “The pen is lifted from three (i.e., their deeds are not recorded): the sleeper until he awakes, the child until he grows up, and the insane until he regains his sanity or regains consciousness.” This was narrated by the authors of Sunan and classed as saheeh by al-Albaani in al-Irwa’, no. 297.
If a child is born lacking mental faculties or brain-damaged, and his parents are Muslim, then he is Muslim too. Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said: “The ruling on one who is insane is like the ruling on a child: if his parents are Muslim then he is also Muslim. This is according to the consensus of the Muslims. The same applies if his mother is Muslim, according to the majority of scholars such as Abu Haneefah, al-Shaafa’i and Ahmad. Similarly, one who become insane after becoming Muslim is also regarded as a Muslim. An insane person who was born in a Muslim society is also regarded as being outwardly Muslim, like his parents or the people of the society, as is also the ruling on children. The children of the Muslims and their insane will be like their fathers (i.e., they will be counted as Muslims) on the Day of Resurrection.”
(Majmoo’ al-Fataawa, 10/437)
Ibn Hazm (may Allaah have mercy on him) said: “As for the insane who have no mental faculties until they die, as we have already mentioned, they are born as ‘haneefs’ (monotheists) and believers, and they do not change their faith, so they die as believers and so they will go to Paradise.” (al-Fasl 4/135).
So if your sister had no mental faculties or power of discernment, and she did not understand the meaning of worship or how to do it, then the pen was lifted from her (and her deeds were not recorded), so she was still adhering to the fitrah, and we hope that she will enter Paradise. But we cannot testify that any specific person will go to Paradise, apart from those whom the Prophet (peace and blessings of Allaah be upon him) testified would go there.
But if she had mental faculties and could understand things, then she is accountable like everyone else, and she will be rewarded or punished for whatever she did after she reached adolesence. If she was fine sometimes and not at others, then she will be rewarded or punished for whatever she did at the times when her reason was sound.
And Allaah knows best.