I remember the first time I collaborated with my mom; I was working as a secretary for a tiny architect firm that had less than 10 people. The business had been started by the father, the mother was the office manager who refused to let anyone else do anything & the son (who was also an architect) changed everything he possibly could about the company any time his parents went on vacation. I worked there for nearly a year while I was finishing up my graduate work & the job ended about 2 weeks before my new job was set to start. I was looking forward to a 2 week vacation, but my mother had very different plans. It was during those 2 weeks that we wrote our first screenplay together & it was the longest and most difficult 2 weeks I've had in my life. Writing started first time in the morning and we didn't stop until we'd met our pages goals/rewrite goals for the day. Although it was a great experience & I was proud of what we accomplished so quickly, I was never so happy to start working again so I could finally relax.
Writing with a family member is a little weird sometimes. Other family members have heard us while we've been hammering out a storyline (which can involve some shouting) and think we're fighting; the truth is simply, when you have two voices that you're trying to blend into one narrative things can get a little explosive. But the best stories are the ones you're the most excited about writing and at the end of the day that's what counts.