Definitely second the bone broth. We started WAPF then Primal because two of my then-three year old's teeth were black & dying. I can now literally chart how much broth she's had by the colour of those two teeth: not enough, they turn black; some, they're brown; a good four bowls a week and I kid not they start turning creamy white. The change can happen as quickly as as four days, so thinner, exposed to the winter elements nails would definitely show signs I imagine.
On the slight off chance it's something more, I'm looking into hypothyroid right now & brittle nails come under a lot of hormone-related problems. Worth thinking about any other seemingly random symptoms that might indicate a wider problem.
ETA: StephenHLi, we cross-posted. But yes, yes & yes to every symptom! I don't want to hijack the OPs thread, but could you explain a little more please?