[pdurrant]

Originally Posted by Falcao

In Spanish, the answers are a) Second nephews/nieces; b) Second uncles/aunts; c) Second cousins

But, since I guess you are more interested in the English terms, wikipedia has a nice article about this.

I rather like second uncles/aunts and second nephews/nieces. You're quite right that they're not terms used in English, but I'm tempted to start using them myself — they seem to be straight forward enough.

               A
               |
        ———————————————
        B1           B2
        |             |
    _________     _________
    C1     C2     C3     C4
    |             |
  _____         _____
  D1 D2         D3 D4

So D1 is C2's niece and B2's great niece and C3's cousin once removed and D3's second cousin

But with the new nomenclature, D1 would be C3's second niece, and C3 would be D1's second uncle.

of course, there is a reason English doesn't have non-generic terms for those relationships: It's rare for any Englishman to actually know his second cousin once removed, or need to specify the relationship.