Bamzors, the only way you are going to be able to legitimately use that copy of the game without violating the copy protection is if you use your brothers Ubisoft account (I.E. he transfers his account directly to you and he never uses it again). You cannot create a new account using your name and his old cd-key, even if he thinks that he legitimately 'gave' it to you. Once that particular CD-key is registered and associated with a Ubisoft account, it cannot be used again. Period.
The reason that he can no longer use his Ubisoft account is to prevent him from logging in to Ubisoft under his account that SH5 is registered to (say, to play AC2 or Settlers) while you are playing SH5...which would be impossible for one person to do, and would immediately raise a red flag with Ubisoft. Once he gives you access to his Ubi account, in principle he has transferred rights to every Ubisoft game that he owns to you as well: One account=one user=one game at a time. Anything that he has registered under that one account will raise a red flag if anything else that he has registered under that same account is in use at the same time.
The reason that I am being so specific about this is that you indicated in your post that you installed the game and tried to create a new account ("
But after the install and registering a Ubisoft account ..."). You
cannot do that. All you can do is use the single, already existing, unaltered account to which your brother has already registered the game. Anything else will not work.
I wish I was wrong on this, but I am pretty sure that your/his/our options are very limited as far as transferring ownership/rights is concerned, according to the EULA that we all agree to everytime the game is installed.
I hope the above info helps clear things up!
