Is it permissible to delete a voice message sent to a father from one of his sisters if it would grieve him, out of fear for him and to avoid informing him of it?

If it is highly probable that the father's得知 the message will cause harm, then do not inform him of it, in order to avert harm, based on the principle "harm is to be removed." And if your aunt asks you about it, inform her using tawriya (equivocation) that you did not receive a message from her, meaning after it was deleted; for ma'arid (allusive speech) is a way out of lying. And if you can handle the matter indirectly to preserve the affection between the father and his sister, that would be good.