Good luck with this. It must be very frustrating to see this happening. Make sure you cross your 't's and dot your 'i's before going to court on this and offer them to settle first assuming you have hard proof that they copied your work product.

Document everything.

They item would be whether or not YC is aware of this and whether or not your application or parts thereof somehow made it to these people. If it did you have a much stronger case, if it did not then you need to document very carefully which bits you believe have been copied, timestamp the works and make sure your lawyer gives you a fair assessment of what to expect in terms of costs and what your claim will be.

I can't look into your walled but depending on how much backing you've got you may be able to hit the pretty hard or you may have to walk away from it, even if you have proof, hard to tell.