Sick pullet, what could be wrong?

I hatched a lavender orpington in May of this year. She was stuck in the egg and required assistance, she was weak for about a week after hatching, but she seemed well after that.
Around 12 weeks I noticed she wasn't moving around quite right, and it seems she's stopped preening.
If she pecks at the ground she falls over, if she changes direction she'll drop a wing for support, her comb is purplish and she has developed what I only know as angel wing on both sides. She's now 16 weeks old, has lived with the same flock since hatching(all hatched the same weekend) and they all free range.
Anyone know what could be wrong with her?

I've noticed in several of my chickens (and a rooster) over the last ten years or so that blue or purple combs (or wattles) seem to indicate a weak heart.  I've butchered some of them after they die on their own and have found over-sized or even 'exploded' hearts.

I'm really sorry your pullet is sick, but it may be that she is just too weak to survive.  I hate saying that.  I'm tearing up just typing it.  But, if you baby her, at least she can be happy.

Thanks! my main concern was mareks I know that is still a possibility. I may end up having her euthanized and sent for testing.
If it were mareks wouldn't some of the other 14 developing symptoms as well?

That might be the kindest thing to do.  I've babied a few chickens and they never got better.

I've never seen Marek's, but a quick web search tells me it probably isn't Marek's.  But I am NO expert.

Best wishes.