User blog comment:Godisme/Panic at the Dollhouse/@comment-44061-20110203012236/@comment-1393830-20110203012632

I have to go back and check the Ju-Ni version but Mangastreams version of The Six Fullbringers did not say it had to be the mother. It just said parent. So by that logic, it is very possible that Ichigo can use Fullbring since Isshin, being a Shinigami, in all likelihood has been attacked by several Hollow

Edit: I checked Ju-Ni and they are the same. Their translation says "Each of us had a parent who was attacked by a Hollow before we were born". So Isshin is the key here I am guessing.