Hello there, all!

I just signed up, so why not jump right in. My partner and I are seperated by many miles and though we know eachother we have never been physical. During our distance we've gotten to explore a lot of secret fetishes through the safety net of the internet before reuniting in person.
We've discovered we both have a pension for role-playing, and more specifically incest role-playing.
I am incredibly turned on by the concept and prospect of this but am at a loss for how to extend out his favorite idea for sexual purposes.
He very much wants me to role-play as his mother figure and older sister who catches him masturbating and shows him the proper way.
Neither of those are great jumping off points for how to last more than thirty seconds of "Nicholas! I can't believe I caught you doing that..." Then it's just like...well, I'm your mother...Yep. I'm your mom.
I suppose I'm just not creative enough 'nor have I ever done anything like that so I just don't know how to play this out for him.
Any suggestions would be great as I'm brainstorming for the fantastic reunion we'll have in a few months and would love to be able to fulfill his fantasies.
Thanks!