My sister-in-law won't reply to any of the requests to finish an already purchased headstone for my brother What can we do?

If she is not responding to phone calls, texts, emails or things of the such. I propose going and seeing her when you know she'll be home or when you know somewhere where she'll be. That way she'll have a harder time evading you if that is what she's doing. And then just ask her what's going on.