I have two watches, a S3 Frontier and an Active 2 both by Samsung. I keep them on my alternative account that I use for my own personal wearing. Almost all my watch faces that I make are on the account I’m posting this comment with. The reason for two accounts is because when I was testing a watch face both my watches would update to the face being tested. By separating the accounts I can test with one and not change the one I’m wearing. If you give your brother your username and password to test the new face with, it will switch your watch also at the same time. I have a couple faces made but not published with my personal account and just three published faces. With this account I have over 500 published faces.
Like I said, I would just publish the one for your brother and name it “Testing” or some other name and in the description state that it will be changed and suggest that people not sync it yet. BTW, they won’t pay any attention and will sync it anyway, but that’s on them if you delete it or change it afterwards.