What should the questioner do to expiate a heinous sin she committed in her adolescence against her sick brother who was unable to defend himself, and who has since passed away, given her feeling of perdition?

The questioner has committed a heinous sin and a great evil. Allah accepts the repentance of those who repent. Therefore, he must sincerely repent, increase his supplications for forgiveness (istighfar), and perform good deeds, for regret is repentance. He must also beware of despairing and losing hope in Allah's mercy.