James and Sirius had made a magical contract back when they graduated from Hogwarts. They wanted their descendants to become joined and become real families, and Sirius also wished to bring in new blood into his blood line even if just to drive his mother up the wall. Of course there were certain conditions that had to be met between perspective descendants in order for the betrothal contract to be activated and come into effect. They had never expected it to be fulfilled in their lifetime as was often the case with betrothal contracts with such conditions like the ones they made. Years later they would regret it.
Little Rosa had survived through the battle and Dumbledore had defeated Voldemort with the help of the Order. She grew up as a happy child and went to Hogwarts with her best friend Neville. She had grown up into a young woman with vivacious charms.
She had a group of good friends consisting of Neville, Hermione and Fred and George.
She had a crush on Fred since she started Hogwarts and finally they started dating in her fifth year. They dated for a year before they broke it off when they naturally grew apart. They remained best friends.
The trouble started when she turned seventeen. It was just after she returned home from finishing her sixth year at Hogwarts. The morning of her birthday the party was started and was well underway when she received a notice by owl from the department of records at the ministry.
It was a letter of notice informing her of a betrothal contract involving her coming into effect. She had dropped the letter horrified. Her mum Lily had appraoched her, picking up the letter from the floor. As she too read the entire contents of the document she whirled round on her husband, who was watching them with half-worried and half-confused gaze. He took the letter from her hands and read through it with Sirius looking over his shoulder. The two men responsible for the mess were horrified into silence, shocked to the core. The simple harsh truth of the situation was this, the Potter-Black betrothal contract had been activated and Lord Sirius Black and the heiress Rosa Potter were betrothed to be married within the year.
Rosa had gone into shock and had locked herself in her room, too shocked to even show her anger or even look at her dad and Sirius. As for Lily, she had become so furious that she kicked out James for the night, and so he stayed over at Sirius' place. In their own right, they were too shocked to speak. This wasn't what they had in mind when they wrote up the binding contract. They went to Dumbledore to ask for his help in finding a loophole or anyway to undo the contract. He had looked at them gravely and eventually agreed to do his best to help.
The fact of the matter was this. The nature of the original contract that they had written up was meant as a binding, non-freeing or breakable by any means other than their personally selected unforgiving circumstances brought on by either of the two people bound in the contract. Those unforgiving circumstances were hurtful ones that they couldn't even consider it even if to break the contract. The goal of the contract was to bind the two people in marriage by soul as well as body and mind, and of course by law. So it was not just a farce fake marriage by name only. Over time their souls would weave intricately with the other's soul and have the benefits of such a magical bind. It was a contract made in the old times by families of old customs, by families who arranged their children in marriages or by lovers who were so sure of their love for each other. It was not to be attempted lightheartedly. If done by true lovers, it was the dream of all lovers to be bound in such a marriage. If done not in this sort of circumstances, it was another thing entirely.
Rosa had finally come out of her room by dinner time the next day, because - truthfully - she was awfully hungry. She had refused to look at anyone, for it would mean she would have to face the reality, even if it was the only thing in her mind. She only half-listened to her mum's attempts to talk to her, half-listening to weak explanations about dad away researching about ways to figure this mess out. It wasn't the news she was hoping to hear, so as soon as she was done eating she left the table.