Disclosing secret to prevent possible harm
11-10-2017 | IslamWeb
Question:
Assalaamu alaykum. My sister asked me yesterday for advice on how to get a loan of 2 million riyals. I was extremely shocked by this request because loans can quickly spiral out of control into massive debt. She told me this in private, and I do not remember exactly, but she probably also told me that I cannot tell the rest of our family. However, I told my mother that my sister asked me something that has me extremely worried, something that could ruin our family. I did not tell her that my sister asked me for a loan, but I told her that I was worried, and now my mother is extremely worried. My mother is demands that I tell her what my sister asked me, and she is upset with me. She told me that because this has to do with my mother and sister, it is alright for me to tell her. The problem is that I am worried that what my sister told me counts as an Amaanah (trust) and that it is forbidden for me to share that knowledge. I do not know the ruling myself, but can I share this knowledge with my mother? She is extremely worried and upset, but I do not wish to share this information if I am forbidden to do so.
Answer:
All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
A secret is a trust (Amaanah), and disclosing it is a breach of trust, especially if the person who confided in you asked you to keep the secret. It is impermissible to disclose it; Jaabir 
narrated that the Prophet, sallallaahu ʻalayhi wa sallam, said, “If a man says something and then turns away, it becomes a trust [which should not be disclosed by the one who heard it].” [Au Daawood and At-Tirmithi] Ibn Sallaam 
said, “The Prophet, sallallaahu ʻalayhi wa sallam, described it as a trust although the one who heard it was not asked to keep it a secret; how then would it be if he was asked to keep it a secret?” [Ghareeb Al-Hadeeth]
Al-Hasan Al-Basri said, “It is a kind of betrayal that you disclose your brother's secret.”
However, if there is a preponderant benefit, then there is no harm in disclosing it. So if you fear that she (your sister) would incur harm upon herself by taking this loan and you hope that your mother would help you deter her, then there is no harm in telling your mother about it.
If this loan is based on interest, then it is impermissible for your sister to get it and it is pure harm without doubt, even if it is through Muraabahah (cost-plus financing) or similar lawful financial transactions. Although such transactions are legitimate, the Muslim should not incur debt except if there is a valid need. Taking such a huge loan often involves incurring harm upon oneself, such as being unable to repay it.
Allah knows best.