It's generally not appropriate behavior in most social and relationship contexts. Sitting on someone else's lap, especially when married, can send the wrong signals and may make one's partner uncomfortable or jealous.
It could be positive. It might be a sign of a close - knit family where such physical closeness, although a bit improper in some views, is just a moment of casualness and shows that they are comfortable with each other.
It could be a rather awkward situation. In the story, perhaps the stepmom was trying to share an exciting view out of the window while on the journey and didn't think about the inappropriateness of sitting on the lap. It could also lead to an uncomfortable conversation later where the boundaries are re - established.
One thing that can happen is that you two could start singing a song together. Maybe it's a nursery rhyme she learned at school. As she sits on your lap, she'll be more comfortable and confident to sing out loud. You can join in and have a great time making music together. Also, it gives you a chance to bond more deeply.
In a 'wife sits on lap story', perhaps they are reminiscing about old times. They might be looking through an old photo album together, and as she sits on his lap, they laugh at the funny memories from the past. It's a sign of their comfortable relationship.
Once at a family gathering, there were no more seats left. My little sister, being the cheeky one, just plopped right down on my lap. We then started sharing a plate of cookies, and I told her some funny jokes. She giggled so much that it made everyone around us smile too. It was a simple but very enjoyable time with her on my lap.
At Christmas time, the family was gathered around the tree opening presents. My little sister got so excited that she ran over and sat on my lap to show me the new doll she got. We then started making up stories about the doll, like it was a princess from a far - away land. It was a magical moment filled with the joy of the holiday and the closeness of siblings.