The Moriarty brothers were clearly trying to set up the elder orphan ("William") as the legitimate second son of the Moriarty family (replacing the true William), while passing off the actual second son as one of the adopted children who died in the fire with the rest of the family. But if you compare "William"'s appearance to that of Albert and Louis, it seems that it should be fairly apparent that he is blood-related to Louis, the adopted child, rather than Albert, the legitimate child. How is this deception even possible?
People might well have assumed that a couple of Earl Moriarty's sons - including the 'adopted' one, Louis - were his illegitimate kids by a mistress and were just too polite to say it. So they may have thought William and Louis probably were in truth biologically related.
How many people died in the Moriarty Manor fire? Albert's parents and his little brother are a given, but there were servants in the house who would have called the children out in the "William" lie (especially the butler). Does that mean that for their plan to work, they had to murder the entire household staff as well? If so, this could easily set the death toll in the double digits.
They did murder the entire household. 9 servants, the parents and the younger son, so double digits indeed. And in order to make sure none could escape, they planned for the whole manor to burn down quickly and from several spots at the same time.
Every resident in Moriarty Manor at the time of the fire perished, but there must have been many other people who were visitors to the house, or knew the family, and were very familiar with what the children looked like. Surely there must have been somebody around who would know that 'William' wasn't the real William? And—unless the two orphans had been hidden away since Earl Moriarty adopted them—there must also have been somebody who'd met them and who would instantly have identified 'William' as one of them.
William had been introducing himself as "William" as Albert's biological younger brother for an entire year in public to strangers after he was adopted to convince everyone he was who he said he was. Meanwhile, the original William had been homeschooled and actively refused to engage with anyone he saw beneath him—everyone—or leave the house to do any errands. Basically no one knew what the original William looked like, and they certainly did not have anyone familiar enough with his face to remember it that well. Anyone who recognized William from before then had been carefully removed from the location and avoided.
I don't see why it was even necessary to give William his false identity. Why didn't Albert just say that his brother William had died in the fire and he and the two adopted boys had escaped?
William taking on the identity of a biological lord gave him much greater access in society to enact his plans. Given the way Louis is treated very differently as the known adopted child and left out of public events William attends and manipulates people actively at, the deception was necessary.
