It's complicated.
The default position with a jointly-owned account (or any other asset) is that, when one co-owner dies, the other becomes the sole owner of the account, and the funds in it. This is so whether the co-owners are husband and wife, parent and child, brother and sister, or anything else.
But this presumption can be overridden:
(a) if the account is expressly set up on different terms (and only the bank can tell you this); or
(b) if there is evidence that this was not the intention of the parties.
For example, my (very elderly) mother and my brother have a joint current account. All of the funds in that account come from sources that belong to my mother, and the account is exclusively used to pay her expenses and to make purchases on her behalf. It's a joint account purely so that my brother can manage all this on my mother's behalf. The purpose for which the account was set out, they ways in which it is used, the sources of the funds that are in the account and the fact that all this has been discussed between my brother and his siblings are all evidence that the funds in the account belong solely to my mother and, when the time comes, they will be administered as part of her estate.
In practice, making a special arrangement with the bank when the account is opened, and opening it on special terms, is relatively rare. Most joint accounts are opened on the basis of a shared understanding between the accountholders and, often, wider family, but the bank isn't part of the shared understanding and it's not reflected in the terms of the contract between the bank and its customers.
So, it's impossible to give a categorical answer to the question raised in the OP without knowing the answer to questions like why the account was set up as a joint account, where the funds in it come from, what they are used for, etc.