There has never been any code that denies 2 family members from having council positions in the same realm.
To clarify, one cannot and never could hold government positions in different realms on the same continent (save for a few cases over the years due to bugs or unexpected code behavior), but one can have two different characters have government positions in the same realm.
The game does add a ton of complications, though, depending on the government system, to get more than one government position. For example, while in some government systems the ruler can appoint himself to anything, in others he can't even appoint his kin.