Patrick Proy started an Open Source Pattern Making project in 2012. While it is currently (and maybe definitely) in pause, it is definitely of great value to look at his development ideas on the web. The project is in java and you can currently still have a look at it here : couture.proy.org/software_en.html
Screenshot of “Couture”. Display cuts off some parts, but you can see the idea and functions of the application.