True, with Ducky's siblings they have shown some that do look exactly like her, but they are never named, nor given individual voices. At most you hear their voices as background sort of babble so it's hard to get them confused with Ducky. I've not seen 12 so no idea about how they do Petrie's siblings there, but in the tv episode and at the end of the first movie I think they do look different from Petrie, or different enough to not get them confused.