I think maybe you're right there lurkette. I think I'm finding it hard to reach any sort of middle ground here, and it's all leading me to the realization recently that I'm sticking my nose too far into this.
The most I should be doing here is lending an ear and a helping hand when she asks for it, and nothing more.
This is all a matter between my girlfriend and her family, and I only have any rights when it's a matter between my girlfriend and me.
My original intent with this thread was to work out whether I should have butted in when her father yelled at her, and thanks to everyone here I've got my answer. The rest of the stuff I've brought into this thread isn't really my business at all.
Thanks again to everyone
I feel much more assured in all of this now