He might break family alliances by publicly opposing the values that these families stand for. For instance, if a family is known for its strict pure - blood ideology and Harry speaks out against blood purity, it could be seen as a break in the alliance.
One possible consequence could be that Harry becomes an outcast in the wizarding world. Family alliances often carry a lot of political and social weight, so by breaking them, he might lose the support and protection that these alliances offer. For example, if he breaks an alliance with a powerful pure - blood family, other pure - bloods might shun him.
Another interesting alliance could be between Ron and Keroberos. Ron, with his love for food, might be constantly bribing Keroberos with treats to get information about the Clow Cards or to ask for his help in some comical situations. Keroberos, being a bit of a glutton himself, would be easily swayed by Ron's offerings.
Perhaps the fanfiction focuses on a parallel universe where Snape, Harry and Lucius are all on the same side from the start. They are part of a secret group that is trying to reform the wizarding world's education system. Their different perspectives - Snape's strictness, Harry's compassion and Lucius' cunning - combine to create a powerful force for change.
It could be about the character Harry experiencing some sort of emotional or physical breakdown within the context of fan - created fictional works. Maybe he's facing extreme stress, a personal crisis, or some difficult situation that causes him to break down in this fanfiction world.
It could be about Harry Potter finding a way to break or disrupt the magic in the wizarding world. Maybe he discovers a powerful spell or artifact that has the potential to shatter the existing magical order.
Well, perhaps in the fanfiction there is a power struggle within the alliance. Harry, being a central figure, might not want to be part of a group where there is constant jockeying for power. Also, new revelations about the alliance's past actions or connections could make Harry want to distance himself. For instance, if he finds out the alliance has some shady dealings with dark wizards in the past, he would surely break the alliance to protect his own integrity and that of his true friends.
One possible plot could be that Harry discovers some dark secrets about the alliances. For example, he might find out that an alliance is being used by a dark wizard to infiltrate Hogwarts. So, in order to protect the school and his friends, he decides to cancel the alliance.
Maybe the main antagonist is a dark force that has been slowly taking over the alliances from within. Harry notices this and decides to cancel the alliances to prevent the dark force from spreading further. This dark force could be something like an ancient evil that has awakened and is using the alliances as a way to gain a foothold in the wizarding world.
When Harry breaks the cursed fanfiction, it might first cause a shockwave through the story's plot. The events that were supposed to happen due to the curse no longer occur as planned. For example, if the curse was making certain characters act in a particular way, they might regain their true selves. It could also potentially expose the source of the curse, leading to a new adventure for Harry as he tries to find out who or what was behind it all.