I’m a naturalized citizen of over 12 years, retired military 20 years married to a American spouse for 30 years. Applying for a Security Clearance. I was asked to provide SSN, eligibility and employment information for my sibling 4 sisters and one brother. What forms would I need to standardize the collected information?
If they are US citizens - additional information is not normally required (special programs excepted).
If they are naturalized US citizens - a copy of their naturalization certificate greatly help you with the questionnaire and any follow up questions.
If they are dual citizens (US/anyone else) or foreign citizens, you will be expected to provide the information requested for any foreign national.
Before anyone pounces - the dual citizenship is complex and the issue can branch out quickly or stay fairly narrow - you still need to report the dual/foreign citizenship of any spouse, former spouse, family member, or extended family member that you have close/continuing contact.