What is the Islamic legal ruling on a son recovering his money that he sent to his father during his lifetime for safekeeping and investment, given that the father has passed away, and the mother and siblings deny this, despite the existence of written proofs?

The issue has some details:

1. If the money was for the purpose of development and safekeeping: The money held by the father is considered to be in the local currency because he was an agent for its conversion, and it should be returned to you in the local currency. If the money was damaged due to his negligence, he is liable for it, and it should be taken from his estate in the local currency.

2. If the money was a loan: If there was no agreement to convert the currency, then the debt is in the foreign currency and should be repaid in kind. If there was an agreement to convert it to the local currency, then the debt is in the local currency.

It appears from the question that the money was sent for development and safekeeping, and that the father used to convert it to the local currency with the son's approval. Therefore, the money is established in the local currency. It is advisable to observe kinship ties and to be gentle when requesting what is rightfully yours.