we'll the title explains it all.
Juste was 18 during Harmony of Dissonace which takes place in 1748 while Richter is 19 during Rondo of Blood which takes place in 1792.
this would mean Juste had to be 43 when Richter was born.
it think its pretty obvious he is either his Grandfather or Father but i'd like to know which do you think he is