Dori, Nori and Ori were all descendants of the House of Durin, though not from the direct royal line that led to Thorin. Their relationship is not explained in the canonical texts, but notes reproduced in The History of the Hobbit reveal that Dori and Nori were brothers, and that Ori was their cousin.
This is based on the fact that Fíli and Kíli were said to be the youngest of Thorin's companions by some fifty years. The elder of the two, Fíli, was born in III 2859, so Dori must have been born in III 2809 at the latest.
2
That is, 'borer' in the sense of a user of boring or drilling tools, not someone who tells uninteresting stories!