This is certainly an interesting topic. I think You answered most of Your questions Yourself already. :) I think the same. For some there is a closer bond than for others. And the storyline can only show so much. Who knows how the characters interact "offscreen"? Maybe there are deeper bonds between some than we know. Maybe it is just not that important to them who is related to whom by blood bonds. Where Scouter and Nightfall are concerned I think part of their never mentioning being cousins is that it was added later to the story. And we never saw them very much together and interacting. After all all elves share orgins from small starter groups. Thanks to their different genetics it does not matter much how closely they are related to have healthy offspring. The Sun Folk are best proof of it. And after hundreds and thousands of years I think relationsships change and lose importance. As Tyleet and Nightfall said in DreamTime 5 they became more like friends and less mother and daughter over time.