Am I sinful for my friend accessing my brother's email, which contains sexual images and videos, after I gave him the email to send me a picture on it?

You bear no sin for what happened because you were unaware of the presence of the forbidden images and did not intentionally show them to others. However, you must advise your brother to cease the forbidden spying on the private lives of others, as Allah the Almighty said: "And do not spy," and the end of the Hadith (definition) states: "Indeed, whoever seeks out the faults of his Muslim brother, Allah will seek out his faults, and whoever Allah seeks out his faults, He will expose him even if he is in the privacy of his own home." You must also admonish your brother to repent from forbidden acts, from forbidden gazes, and from possessing forbidden images, and remind him of the Last Day.