I think tinnu morafon nailed it !
some verbs like be, see, know, have are not commonly used in the continuous tense. However, the exception for it is if the state is temporal
"I have a brother" means one thing, and "I am having a brother today" means something else (Probably my mom is giving birth right now). Moreover, "You are silly" and "You are being silly" are different in the same manner. One is a temporary state.
The question of if this is academically accurate: yes, it is. Not as common as other tenses, but it may occur.
If it is used in the past, or the past perfect, then the nuance in its meaning would carry the same effect
Great question ! loved it !
I hope I helped somehow