What is the expiation for backbiting a deceased father, and how does one deal with siblings reminding them of that?

You erred by backbiting your father, and backbiting is forbidden; it is even more severe when it concerns one's parent. The expiation for this is sincere repentance, frequent seeking of forgiveness, praying for your father, and giving charity on his behalf. As for your siblings, remind them of Allah and inform them of your repentance, and that Allah accepts repentance from His servants. It is not permissible to taunt a Muslim for a sin from which they have repented.