As far as I am concerned I only refer to 'my men' or those who are under my command, hence why he informed Firenze about his battle beforehand. My men followed me there, (they start betting), so naturally I'll address my men after receiving my marching orders.
If you have soldiers there aswell they'll have their own captains so they'd ignore my orders especially is there not vanguard.

Since the Vanguard often were made up of elite forces (it being a position of honour) I went on to assume that my battalion basically exists out of Knights. 
The soldiers there would not fall under me and thus would not be feeling addressed by his call 'his brothers & sisters'.
I've changed the ranking sergeant to senior knight.