What is the ruling of Islamic law on burying a person in the grave of my recently deceased father without our knowledge, and are we sinful for not objecting at the time?

There is no sin upon you for what occurred, because it was not by your action, and no soul burdened with sin will bear the burden of another. Indeed, he who gave permission and buried another person in your father's grave made a mistake, and it is forbidden to exhume a grave to bury another person in it before the first deceased has decomposed. You are not permitted now to exhume the grave a second time to remove the one who was buried with him.