I am repeating my reply to make it firm that you have to give details of all the siblings. In case you do not have their addresses or they live in another country, you may only mention the city and country name in address column. Their names or personal record is not related to you.
This is an important information which is required by CHC, you can not conceal or avoid. It has an impact on your process for misrepresenting, it can cancel your immigration/citizenship at any stage of your life.
Best regards,
Man J.