How you feel is how you feel, HIA. Happy birthday and anniversary to you both.
You know there will be other anniversaries, and that you can celebrate later. But her dad is her *only* dad. I would be with mine too under the circumstances. She is where she should be.
Doesn't make it suck any less and it certainly doesn't mean she loves you any less.
But you already knew all this. I hope her dad/your FIL pulls through ok.