On Shadowhunters.com under "The Last Hours", Cassandra Clare stated that she will be making a sequel series to The Infernal Devices.

Here is the description:

The Shadowhunters enter the Downton Abbey era in The Last Hours. Will Herondale is head of the London Institute, where he lives with his wife Tessa. But now it's their children, James and Lucie, who take center stage, alongside their parabatai, Matthew Fairchild and Cordelia Carstairs. Begin their adventures in Chain of Thorns.

There will be 3 books, Chain of Thrones, Chain of Gold, and Chain of Iron.

I can't wait!!