There is no reference in physical, picture reference to their non-combative apparel, but apparently (and I'd need to check the source a bit more) they wear what are called "Shield Robes". Shield Robes are basically nun habits/robes that are made of an armoured mesh which protects from small arms and melee strikes (basically las and stub guns), and it's said that while the Non-Combative Sororitas wear them basically 24/7, the Orders Militant/Battle Sisters wear them when not required to wear Power Armour as they're easier to train and pray in than the bulkier Power Armour.



In my opinion though, you've basically nailed a "Shield Robe" that the Sororitas would wear.