When you say someone is guilty of doing something wrong you make an accusation, like your accusation that your brother used your computer without asking first.
Accusation comes from the verb accuse, which means to charge someone with a crime. It is important to remember that an accusation comes about when someone thinks another person has done something wrong or committed a crime. It doesn't mean, however, that the person is guilty. There needs to be proof — an investigation or trial, even admission of guilt — before a conviction, meaning the person really is guilty.