Yes. USCIS Form I-730 instructions address situations in which primary evidence of a family relationship is unavailable and describe secondary evidence and, in some circumstances, affidavits. The exact evidence depends on the claimed relationship and why the primary record cannot be obtained.
Start with the expected primary evidence
Identify the civil document USCIS normally expects—for example, a marriage or birth record. Then document whether it exists, whether you requested it, and why it cannot be obtained if it is unavailable.
Do not jump directly to secondary evidence without explaining the missing primary record when the instructions require evidence of unavailability.
Organize secondary evidence by what it proves
Possible records can vary widely. Create an index that states: document, source, date, whose names appear, and the relationship fact it may support. Keep original-language records with complete English translations and translator certifications where required.
For a marriage, evidence may need to address the legal relationship and, depending on the issue, the history of the relationship. For a parent-child relationship, the relevant secondary evidence will be different.
Affidavits are not a substitute for careful preparation
If affidavits are used under the applicable instructions, the writers should state facts they personally know, not repeat a script. Record how the writer knows the family members, what events the writer personally observed, and how long the writer has had that knowledge.
Never manufacture a record or ask a witness to state something they do not know.
Build a missing-primary-evidence note
Keep together: expected primary document, attempts to obtain it, proof of requests or unavailability, secondary evidence, translations, and questions for counsel. This makes the issue visible rather than burying it inside the packet.
Official source
Use the current Form I-730 instructions immediately before filing because requirements can change. This page is educational information, not legal advice.