With the amount of information given by you, it will be difficult to comment much on the your sisters health. You have mentioned that she has stayed isolated and did not understand much and also cannot take care of self. Your description hints of your sister having some kind of mental sub normality. However such labels can only be substantiated after a thorough testing by various psychological tests. Recent change in her behavior to crying episodes, irritability and excess sleep can be a pointer to a fresh depressive episode. But as i said earlier, the mount if information given by you is too small to comment anything significant. You need to consult a psychiatrist
to get her assessed in detail.