Does sin accrue to me for being unable to attend my brother's washing (ghusl) after his death, and what must I do?

There is no sin upon you for not attending your brother's washing if someone else is available to perform the task. In fact, the jurists have stated that those whose presence is not needed for the washing should not attend. Only the washer and those whose help is required should be present. It is recommended to frequently seek forgiveness for him, give charity on his behalf, and perform Hajj and 'Umrah if possible.