That's a tough question, tbh. Generally, it's by being somebody the petitioner or applicant knows well (like a close family member). Basically, they need a real reason to support the individual. For example, people have mentioned - just as an example - an acquaintance. They technically qualify as a sponsor, although they have no real reason to want to support the individual so they are not a good choice. if you are close to the aunt, then I doubt that will raise much of a concern.
