Does informing one's sister about the experience of severe pain during intercourse at the beginning of marriage fall under the warning of "divulging a secret"? What is the Shariah ruling regarding discussing intercourse between sisters, and is it entirely forbidden, or is it permissible for the purpose of guidance?

The Sharia has forbidden the disclosure of marital secrets. Abu Sa'id al-Khudri reported that the Prophet (peace be upon him) said: "Among the most evil people in the sight of Allah on the Day of Resurrection is a man who goes to his wife and she goes to him, and then he spreads her secret."

An exception to this prohibition is when there is a need to clarify a Sharia ruling, offer advice, or resolve a dispute between spouses. This is based on what was reported from Aisha (may Allah be pleased with her) that a man asked the Prophet (peace be upon him) about a man who has intercourse with his wife and then becomes sluggish (i.e., does not ejaculate), whether ghusl is obligatory for them. Aisha said: "Indeed, I and this one (meaning the Prophet) do that, then we perform ghusl." Also, there is what was reported in the story of Rifaa and his wife.

When there is a need, it is preferable to allude rather than state explicitly, and to mention the matter generally rather than in detail.