There are several reasons, but none of the answers so far list the most important: legal obligations and disclosures.
For large companies (and sometimes medium or even small companies) with government contracts, public shares, contests or legal disclosure requirements to other companies (partners, clients, etc.) your blood relations - and their places of employment - can have a legal impact on the contracts, contests, disclosures, etc.
For example, if you have an uncle that works in the aerospace industry and you get a job with a government contractor. During your employment, your employer may ask you if you are available to work on a contract that you then tell your uncle about during a holiday family event. Harmless as it may have been intended, for all you know you just gave away significant information to a potential competitor for the contract, which can be illegal. You could even end up in jail for something you did not know was illegal (how often have people "claimed" that they didn't know??).
Or you are trying to get work on a the distribution line of a new tech product, and your aunt just "happens" to be the purchasing manager for the public company that your company will be supplying. Get the point?
There are other reasons (such as nepotism or general interest), but it is very unlikely the company is trying to determine your actual genealogy or actually cares if you have family members "helping" you get the job or whatever else you might imagine. They are most likely just trying to cover their legal requirements.
If you are unsure of what they consider a "relative" then ask them. If you do not disclose it to them (because it's "too hard to know" or you thought it didn't matter) and it later becomes a problem, you could end up in very hot water... not limited to just losing your job.