Unless you have told the bank otherwise, none of the funds in the joint account are held discretely by either of you (that is, you don't own "half each"). The partnership owns the total in its entirety. This means that when one of the parties dies the entire amount reverts to the survivor. Neither a will nor the rules of intestacy can change that. So, no, you don't have to mention the joint account in your will because the funds are not yours to bequeath (they belong to the partnership). Your sister, as the survivor of the partnership, will need to provide a copy of your death certificate to the bank and they will change the account to a sole account in her name.