Is what a woman claims, that she visits graves, speaks with the dead, and knows their conditions after death, sound according to Sharia, and what is the evidence for this from the Qur’an and Sunnah?

What the woman mentioned about knowing the state of the dead after their death, her speaking with them, her visiting the questioner's parents and sister, and her describing their state and what transpired between them, falls under matters of the unseen which Allah alone possesses knowledge of. Such knowledge can only be known through revelation from Allah to one of His prophets. No authentic hadith has been reported from the Prophet (peace be upon him) regarding this. Inspiration or dreams are insufficient for knowing such matters, as they can be both correct and incorrect, and to claim or rely upon them is to cast stones into the unseen.