A.T. Robertson Commentary Luke 16:28

"for I have five brethren; that he may testify unto them, lest they also come into this place of torment." — Luke 16:28 (ASV)

That he may testify (οπως διαμαρτυρητα). An old verb for solemn and thorough (δια-) witness. The Rich Man labours under the delusion that his five brothers will believe the testimony of Lazarus as a man from the dead.