Has it ever struck you that most uncles and aunts seem pretty detached from their nephews and nieces? The duties are not well-defined. The impetus to help raise the nephews and nieces is mostly obliterated by a sense that it might be seen as interference (and surely, sometimes it is!).
But it seems to me that a proper understanding of the covenantal structure of the family would place uncles and aunts in a much closer position to help than, say, daycare. After all, uncles and aunts are descended from the grandparents of the nephews and nieces. Maybe a slightly broader (or deeper!) view of the covenantal structure (not headship, obviously, as the father is the head) would dictate a closer involvement.
Most relationships between uncle/aunt and nephew/niece seem to rely mostly on the nephew/niece initiative. Why should this be? Is there no telephone? Is there no internet? Is there no such thing as gifts? I can testify that every time I have gotten involved in the lives of my nephews and nieces, not only have the parents been grateful, but also the nephews and nieces have been grateful. Our culture is rootless enough as it is without this fragmentation of the larger family that tends to happen.
I believe that the Bible hints at a larger involvement. One could profitably look at the house structure of families (one room added on whenever a new family starts), and the importance of uncles and aunts (even in a negative light, such as Jacob and Laban) in the biblical narrative.
I would therefore like to challenge us to be much more involved in the lives of our nephews and nieces. This is especially true of those nephews and nieces that become ours by marriage. They are not to be treated in any different way than the blood-line nephews and nieces.