Final answer:
The case manager needs to provide official documentation to prove the relationship between a sponsor and a UAC who are related as first cousins once removed. DNA tests could also be used to demonstrate a shared genetic heritage if necessary.
Step-by-step explanation:
The question posed concerns the legal proof required to demonstrate a familial relationship, specifically the case manager's duty to prove the relationship between a sponsor and an unaccompanied alien child (UAC) who is the sponsor's first cousin once removed. This would typically involve providing official documentation such as birth certificates or other legal documents that establish the lineage from the common ancestors (the grandparents of the UAC and the parents of the sponsor). In terms of DNA evidence, which is not directly mentioned in the question but related to proof of relationship, a child's DNA fingerprint must have common genetic markers with those of their mother and father to establish a biological relationship. However, for relationship proof between cousins, DNA testing might be used to demonstrate shared genetic heritage. The case of Sharon Kowalski and Karen Thompson, though relevant to guardianship and legal rights of partners, is a separate matter from proving familial relationships and does not directly apply to the case manager's situation described.