Zaid Alsalami, Shaykh Dr Zaid Alsalami is an Iraqi born scholar, raised in Australia. He obtained a BA from Al-Mustafa University, Qom, and an MA from the Islamic College in London. He also obtained a PhD from... Answered 4 months ago

Bismihi ta'ala

If you know that your brother would be ok with you using his amanah in a wise and responsible way, and that you would return it back to your brother once he asks for it, then you are allowed to lend that money to your father. 

And Allah knows best.