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Short and sweet.

1.  Above all else, this is a ridiculous story.  That's not carte blanche to go off the rails, but as long as you show a little restraint and provide explanations for things that might otherwise be stupid, then by all means go for it.

2.  Keep the characters consistent.  Allowances for Rule #1 aside, the characters should be reasonably similar in each branch.

*Adam is soft-spoken and largely a nice kid.  He's also a horny little bugger who is undeniably attracted to his mother.  He'd never do anything to hurt or (publically) humiliate her.  However, he won't be above taking advantage of his position, especially when they're alone.

*Sarah is kind and accommodating.  She's peripherally aware of Adam's interest in her - she's noticed him staring from time to time, but she writes it off as a phase.  Critically, she's very open-minded.  She'll be resistant to any sexual request Adam makes, but she won't be wholly unwilling and, deep down, would enjoy making her little boy happy.  The money is only a tiny factor for her.

2a. Keep the characters' appearances reasonably consistent.  Minor details like Adam's exact dick size or whether or not Sarah has pubes can be fudged in different branches so long as subsequent branches are kept in-line.

*Adam has light brown hair, blue eyes, is shorter than his mother, and is slim.

*Sarah has dark brown hair, deep blue eyes, is around 5' 6", and is curvy without being fat.

3. Don't go too crazy with the fetishes.  At the very least, try to keep them character-consistent.  Adam isn't the kind of guy to mind-break his mother into being a debased sex toy, for example.  But hey, maybe he has a foot fetish, or maybe Sarah secretly wants to blow the family dog.

Other than that, have fun!