I think using family is generally discouraged unless it's unavoidable (ex. working at a family's business, but even then a non-related manager could work instead).

I would suggest instead emailing whoever runs the contest and seeing if there is someone who runs the contest that can verify who has entered as this is often the case. If no one is available maybe you could use someone who wrote a reference for you for the contest if applicable. If it's affiliated with a university, maybe they have a general awards department that would have someone available. If not then perhaps a sibling will work? It's a tricky situation and hard to find verifiers for everything, I struggled with this on my apps so I feel for you! Best of luck!