I'm not a sewing person, myself. But from a quick bit of duckduckgo fu I just found this website:
I haven't been able to take a proper look at the site, because it requires signing up for a free account - which is something I have no interest in. But once you are logged-in, it looks as if you can start designing your patterns online via your browser.
I don't know how much control you have over the pattern - but from what I've read, it sounds as if they have a lot of different patterns for different types of clothes and you just enter various measurements in order to create a customised pattern that will fit your needs.
The entire website appears to be 100% Free/libre/open source too - so it should be possible to extend their collection of patterns too.
As it is free, it might be worth signing up and taking a look! It may fit your needs exactly. And even if it doesn't, the community there would be better placed to recommend other free-software to assist with pattern making.