This is a series of small interactions based on a plot bunny (included below). I actually don't have time to flesh out the whole story and would love to see it adopted. But I will add on various ideas as I have them.
Serving the House of Potter
As the two boys ate the haul that Harry had bought from the trolley, Ron Weasley nervously looked at the boy he shared cabin with on the Hogwarts Express.
His father had taken him aside the day before and asked him to perform a function that he, Ron, would be in a unique position to perform. He remembered the conversation quite clearly.
Flashback
"Why can't Mum know?" Ron asked.
Arthur sighed. "Aunt Muriel. That's why, son."
Ron was confused. "What does Aunt Muriel have to do with it?"
Arthur took on a grim look. "Muriel Prewitt is the Regent for House Prewitt. Your Mum and I, we married for love. Her dad was happy to allow it, even though House Weasley hasn't been in the best position since your Grandda' and Grams got married. The Blacks weren't happy. So they took back Mum's dowry. When I married your Mum, she brought a decent dowry with her, and we were happy." He smiled in reminiscence. "Your Mum loved me and her dad was only concerned with her happiness. Me too." Arthur grinned. "Your mum even used a mild love potion on me to increase my attention toward her. Didn't work, 'cause I already loved her more than the potion would cause."
Arthur's smile became sad. "When they died, though, Muriel became head of the family and she never liked me. She can't break our marriage, but if she finds cause – she'll disown your mum and take back the dowry. And because the dowry was all we had when we married, taking it back would mean we'd lose everything."
Arthur became grim. "Muriel is using her authority to force your mum to train Ginny to seduce Harry Potter. And I can't stop it because it would ruin us. So, the only choice we have is to do as I asked you to do."
He looked squarely at Ron and said, "So, Ronald Bilius Weasley, will you do as your Father has asked as regards Heir Potter?"
Ron, knowing how important it was, nodded. "I'll become his friend and start teaching him about our world. If I can, I will ask him to take House Weasley as retainers to help him take his place in society."
Arthur looked at his son proudly. "Tomorrow you ride the Hogwarts Express and our future rides with you. You give me great hope, son."