They can be, provided the arrangement is documented as a gift or loan and the family member's own lawful accumulation of the funds is evidenced. What matters is the paper trail, not the closeness of the relationship. Decide early, because assembling a relative's records takes as long as assembling the applicant's own.
Separate the funding question from the household question
A relative who contributes funds does not thereby join the case. Derivative eligibility runs to the investor's spouse and unmarried children under 21, and age protection is fact-specific, so a child nearing that threshold may make a slower funding plan the wrong choice. Set out who is intended to migrate, who is only lending or gifting, and record each intending applicant's country of birth, since chargeability generally follows birthplace.
Hypothetical example: a share of a family holding company is expected to complete the funding, and the contributing relative is treated as part of the case because the money comes from within the family. He is not. A first review would document the contribution as a gift or loan with the relative's own lawful accumulation evidenced, and would separately list who actually intends to migrate.
It would also record each intending applicant's country of birth, since chargeability generally follows birthplace rather than residence or passport, and would flag any child approaching twenty-one, because a slower funding plan is a different decision for that household.