Yes, it's worth adding to your resume if you think it is suitable and relates to future jobs that you apply for. Personally, if I wanted to work in a kitchen as a chef, then yeah. It relates to the job and shows that you've worked in a similar environment in the past.
As for the not getting paid part, it depends where you are in the world as to the laws and legality of that really. Personally, helping a family member out is one thing, but working there for six years is another.
Any experience whether paid or not is a good thing. Future employers don't need to know what you were being paid whilst there although they might want to know. List it as work experience nonetheless. It shows commitment and that you can give your time to help in anyway you can. Plus gaps in employment etc look bad, so this shows you were at least doing something.