Is that last pair of bubbles Tenshi or Iku? The だから makes me think it's continuing from the line before it, which is Tenshi's, but the だけれど is more like Iku's speech.

I wonder if 降誕祭 is supposed to mean Tenshi's birthday rather than Christmas, given that she technically qualifies as a "saint or great man". I also wonder if I could have translated おばさま any more poorly than this.

I think it is more of Tenshi's line for the last bubble, indicating that they should be holding hands. Don't think the confused Iku would think of that.

A birthday seems more likely here. If it's Christmas, they tend to word it out as so.

And yes, "auntie" sounds close enough.

Tough call. I give a slight edge to it being Tenshi, as it seems to follow her mindset a bit more: She's the one who's considered them as "dating", so she's the one deciding it's time they held hands.

The "birthday" line seems about as good a compromise as one might get if one doesn't want to use "Christmas" outright. I'd only suggest including the point that it was "last year". In fact, I'll do that before posting this.