Lily lives because she had triplets. the male of the three died. Lily, Sirius, and Remus raise Harry and the girls. They were born early, so Harry is not quite two years older. Sirius has Harry as his heir. Dorea Potter nee Black makes it legal to the Ministry and Goblins. Remus takes a potion that turns him into a woman. It is because they love each other. Sirius makes sure he is now sterile. He does not want Remus to go thru the partial pregnancy that all weres do. The fetus dies at the third month, because of the change.
Lily has sex with both of them, by the time Harry is thirteen. The girls are always hugging and touching him. They are in Hufflepuff, though the hat had told all three they could be in any house. The hat had put Harry there because he was more loyal than brave, more loyal than ambitious, his intelligence was great; but his loyalty to his sisters, Lily, Sirius and Remus far outweighed it.
By the time of the TriWizard Tournament, Harry knew what was going on and had vanquished Voldemort three times. Since it was defense each time, the Goblins had instigated a rite of conquest. The only problem was he needed to be emancipated or fifteen. Being the fourth champion; with the Supreme Mugwump saying he had to compete; the Chief Warlock also saying it with two heads of Ministry Departments; and the Headmaster of his school, also saying so; made him an emancipated minor.
The thing is, he could now marry anyone, especially with a contract. The thing that changed everything was that as he was virile, the Contract that Drunk James and Sirius did, took effect. Any unmarried Potter female must marry the next lord Black or forfiet their magic. At the time they were writing it, they thought it was unmarried Blacks marry Harry. They did that to get revenge on the other Black sisters and cousins. The details were so harsh, that the girls would want it and never say no to their spouse.
The Contract that Articus did was that any daughter of Andromeda must be married to the Lord or heir of House Black. He had saw the contract that Sirius wrote and copied most of it. Especially the money and obedience clauses. Sirius could not be either now, after his sterility potion. Andromeda was not kicked out. The penalties he set into this contract were magic and life threatening. He made sure, that any female offspring could not do what Andromeda did, unless already married.
When they were in the side room, no one knew what was happening outside it. Rose and Ivy had collapsed. Harry had contracts with Susan Bones and Luna Lovegood so that both could keep their houses name. They also collapsed. Nymphadora Andromeda Tonks collapsed at the Ministry. Lady Bones did a bunch of detection spells and could find nothing. She got a message that Susan had collapse, basically at the same time.
She spoke out loud; "Oh shit, she is in a marriage contract and sense Susan collapsed it means that she must marry Harry Potter."
At the Tonks residence, Belladonna Narcissa Tonks had collapsed. She was just ten. But the contracts did not care.