Which links does Google use for the Sitelinks?
How can you get Sitelinks for your website?
Unfortunately, there is nothing certain about Google's Sitelinks. The following factors seem to influence whether Google displays Sitelinks or not:
1. Your website must have a stable #1 ranking for the searched keyword. Other websites don't seem to get Sitelinks.
2. Your website must be at least 2 years old. It seems that younger websites don't get Sitelinks.
3. The number of searches and the number of clicks that your website gets for a certain keyword seem to be considered. Keywords that aren't searched often enough don't get Sitelinks. It also seems that your website has to get many clicks for the searched keyword.
4. The number of links that point to your website with the searched keyword as the anchor text seem to influence the creation of Sitelinks. Sitelinks only seem to appear for the main keywords of a website, not for all keywords for which a website is listed.
If your website meets these criteria Google might assign Sitelinks to your website for your most important keywords.
Source: Axandra Search Engine Facts