-drops the Hollyleaf subject-
'There will be three kin of your kin who will hold the power of the stars in their paws.'
That was the prophecy basically directed at Firestar. It states that Firestar's kin will be 'more powerful than StarClan'. Now, to begin with, kin means one's relations or family. It doesn't have to be a direct line, like father to daughter to grandson, what have you. All it really means is that three of Firestar's relatives will have the power of the stars in their paws.
Whitewing and Birchfall's kits being included in the prophecy isn't all that far-fetched once you have that knowledge. First, the prophecy doesn't specifically state names or a certain generation; it only says 'kin of your kin'. That could mean any of his family so long as the tree goes on. Second, Whitewing is the daughter of Cloudtail and Brightheart. As we all know, Cloudtail was the offspring of Princess, Firestar's kittypet sister. He was adopted into the Clan; most know the story. But if you follow that generic branch down the line it does whittle down to the kits Dovekit and Ivykit. So, the kits may not be Firestar's direct descendants, but they are still related to him.
Lastly, the reason why Hollyleaf, Lionblaze, and Jayfeather weren't the three, in my hypothesis, is that Hollyleaf was corrupt in a way; we all know what she did, so on and so forth. They really just weren't the ones. Although, one may question that Whitewing only had two kits, and Lionblaze and Jayfeather are still there, so where/who is the third cat to hold the power of the stars? Well, at the end of the book Jayfeather said, and I quote, "We aren't the only kin of Firestar's kin. It doesn't matter which of Whitewing's kits is the one. There are still three of us!"
Once you break all of that down it isn't that hard of a concept to grasp.