Will my sister, who was born with brain damage and died at the age of 16, unable to read or write, enter Heaven or Hell?

Every newborn is born upon Fitra (primordial nature). The insane person is not held accountable. If their parents are Muslims, then they are Muslim by 따라 (following) them. Similarly, an insane person born among Muslims is judged as Muslim outwardly, following their parents or the people of the household. If the sister lacks intellect and discernment, and does not understand the meaning of worship or how to perform it, then she is not held accountable, and she is upon Fitra, and we hope for her Paradise. However, if she is rational, understands, and discerns, then she is accountable like all other accountable individuals, and she will be rewarded for what she did after reaching maturity. Likewise, if she sometimes regains consciousness and sometimes loses her mind, she will be rewarded for her actions during the times she is conscious.