well, he didn't donate it to the aunt, but to her partner and it was her partner that had the child. So he didn't have a baby with his aunt, which is what the title implies.
I think 15 is too young. I think that the aunt was quite selfish to allow her desire for a child to blind her to the fact that her nephew was only a child himself and it was not fair to a child to get him to father a baby when he would have legal responsibilities to that child that he was possibly too young to understand and therefore make an informed decision on. erm. iyswim.
However, according to the article he did it again when he was 18, so clearly something he was very committed to over a long period of time. What he did, no doubt he did because he loved his aunt.
Whether folks feel it was right or wrong or daft or just plain selfish, those children exist, and now his parents have 2 grandchildren. That may well be such a comfort to them.