What does Incest mode provide? There's only 2 sex scenes at the moment right? With Julie doggystyle in different rooms and missionary with Linda. As well as the Christmas gift with Linda.
When you are finished with her other content she will hint that something is bothering her. Keep exercising with the dumbbells in Mom's bedroom until an exclamation point prompt with Julie's face comes up. That means she 'likes' you enough to tell you. This triggers the scene you're looking for. That's what worked for me, anyway.