Both sites are very similar and both use the same assets (screen shots). Even a lot of their products appear to be the same. Most likely they're really the same.
That said, yes it is a GPL violation if they're using Sigil's code and not releasing it. Unfortunately, there isn't very much that can can be done. Some of you might remember that we had this
happen with calibre. My options are 1) try to sue them, 2) publicly shame them into doing the right thing. 1) is costly, time consuming and if they're not in the USA (where I am) it might not be an option at all. 2) This only works the violators actually want to do the right thing.
This is the reality of the situation. While it sucks, it is what it is.