Any drug, if taken in excess can have grave consequences. If your sister has suicidal impulsions, she needs to be taken to a psychiatrist
immediately. Because even if you hide your tablets, a suicidal person looks and finds other means to commit suicide
. She needs professional help. Also, talk to her. Try and find out why she wants to commit suicide. All people having such tendencies are depressed. Try to find out why she is so depressed that she feels the option is dying. Give her affection, attention and consult a professional.
All the best!