User blog comment:Stone Roger/Natsu and Zeref have somekind of relation?/@comment-110.143.150.135-20140705075831

If Mavis is Natsu's mother, then he wouldn't have been born in the Dragon Civil War because Fairy Tail is only in the third generation, after being founded by Mavis in the year, X686. Therefore, even if Zeref is Natsu's father, Natsu wouldn't be around 400 years old. He would only be about 100 years old.

So basically, Natsu is Zeref and Mavis' son but is abandoned by them and picked up by Igneel. And when Zeref learns he is to be a father, that becomes the final trigger his "Value of a Life" leading him to wish for death at the hands of Natsu. (Other triggers include possibly, falling in love with Mavis and later marrying her.)

It is also possible that Gajeel and Wendy are the older brother and younger sister of Natsu while there is some familial relation between the three and Sting and Rogue. Although this is only speculation on my part. But it makes sense, to me, since it seems farfetched that 4 other children, assuming that Natsu is the child of Zeref and Mavis, would be abandoned by their parents. Sure, they could either have their parents killed or be abandoned by them but I don't think it's likely that another family in the village/town doesn't take care of them. And what child would be brave enough to wander around, especially when they won't be able to walk and talk properly, if they aren't even 5 years old when they were first picked up by their respective dragons.

--J.K-Y