The simplest, direct and economical way is to take your ID card to any {consultation center of the Home Affairs Department} in Hong Kong, call 1083 to get the phone numbers of these centers, and then ask for the addresses. You can take an oath for free when you get there. You will verbally declare your relationship with other relatives with the staff there, and he will fill in the affidavit. Later, you will take the oath in front of the oath officer. He will send you this affidavit after signing it.