No. Although who may appear on behalf of a business differs with the type of business, you would have to be at the least a regular employee. A partnership may only appear through a partner.
If your relative owns the business as an unincorporated sole proprietorship, you would not only have to be a regular employee, but the claim would have to be something you would prove with company business records, and you would have to be able to testify about how those records were prepared.
A business other than a sole proprietorship (e.g., John Doe, doing business as ABC Sales) can appear in a California court only through an attorney. Therefore, if the business is a partnership of any kind, a corporation, an LLC, or a trust, it must appear by an attorney.
As to whether you can testify on behalf of the business, it depends on whether or not you are a percipient witness -- i.e. , whether you have personal knowledge of the facts about which you propose to testify. Even a custodian of records of the business must have personal knowledge of the fact the records are records of the business and that they were prepared and maintained, and are relied on, in the ordinary course of the business.
Thus, the controlling issues are the type of entity and your knowledge of the facts, not the relationship between you and the owner of the business. If there is any question about the foregoing, consult with the lawyer who formed the business or other counsel. If the business has no lawyer, you can obtain a referral from the Sacramento County Bar Association Lawyer Referral & Information Service at http://www.sacbarlawyer.org/.
Disclaimer: This answer is provided as a courtesy. This response does not constitute legal advice, which requires an attorney-client relationship, and this response does not create an attorney-client relationship between us. It is impossible to properly evaluate a legal problem without a detailed consultation and a comprehensive review of all the facts, documents, and/or other materials involved. In addition, if you are in a state other than California (where I am admitted to practice), your state may have different laws. You therefore should not rely on this answer, but should consult with local counsel for definitive guidance.
Business entities such as corporations and LLC have to appear via an attorney. You could appear to testify as a witness, but only if you have personal knowledge of the matters you would be testifying about. I concur with the excellent answers by counsel above.
Please feel free to call Attorney Martha Bronson at 209-830-0400 for help. Martha Bronson is an experienced attorney in the Sacramento, San Joaquin and Bay Area courts, having been in practice over twenty four (24) years.
Please note that this answer does not constitute legal advice, and should not be relied on, as each situation is fact specific, and it is not possible to evaluate a legal problem without a comprehensive consultation and review of all the facts and court pleadings filed in the case. This answer does not create an attorney-client relationship.