The De Chagny family did not exist. Ever. The only one family that existed and still exists, are the Carpenter the Changys. Now, we know that Philippe (Raoul's brother) was 41 when he died, so if we search that family's counts and their deaths, we can actually find some clues. One count, Francois-Charles (1791-1837), died at the age of 46, which is close to 41, but the dates aren't exactly correct, I'm afraid. Leroux stated that his story occured in 1881,and this man died in 1837.
That's 44 years. Next generation: Francois's son, Eugene: he was born in 1819 and died 1889. So he's not our guy. The next one was a boy named Jehan, born in 1861 and died in 1942. He actually was the youngest son of the family, just like Raoul. Three generations later (1838), a man called Eric Carpentier, who had a cousin called Philippe, and was never adressed as "De Changy. Obviously Leroux knew about them, and used their names, as the last ones lived at the same time as him.
Note: Did you know that Leroux admitted that he had a crush on Christina Nilsson?