The plan was to appeal to the current Headmaster of Hogwarts, Headmaster Dippet, and be allowed to transfer as an orphaned set of twins from France. To avoid having to make more complex lies, they would claim that they had been orphaned before the actual war, never knowing their parents, and had been staying with a foster family, who had been homeschooling them in lieu of the war, before they was killed by some of Grindelwald's men in a raid. It would take some very complex, and somewhat illegal potions to make them blood siblings, but they had stopped caring for labeling magic as Dark, and therefore out of bounds, when it could be very useful. Now, the reason this particular potion was illegal was because it could be used to steal a title or monies that were not originally yours by blood. One could "join" a family and then kill the heir and head and gain the title quite easily. Seeing as Harry wanted to share his titles and monies with Hermione, they didn’t very much care that it could put them in Azkaban for making.