I've tried doing a few original fiction projects with my sister, but we are both hopelessly bad at motivating each other beyond basic world-building (though we are probably each other's best editors), so they never got much of anywhere. Also, I have a tendency to get possessive about ideas that I have mentally tagged as 'mine' in some form, which means if I am actively trying to write them, it's really hard for me to see them done 'wrong' by someone else, which is the other reason those attempted collaborations fell apart; Vicky and I construct stories along notably different patterns, and we got into intractable arguments over plot and character arcs.

But I have done some world-building for Vicky that I then handed over to her as a base for her own stories, since she isn't very detail-oriented in that particular way, and I had no problem working in icedark_elf's mercverse shared FF7 AU. My guess is that if I'm starting from someone else's idea, it's easier for me to relax, and also that it's easier for me to see someone else use my basic ideas in her own story than to let someone else co-write a story I see as 'mine.'