There are a few large issues to be considered here.
I have just started the process of dealing with the accounts of two elderly relatives.
In one case, I have taken the approach of being attached as an authorised signatory on all accounts. The process was quite rigorous : i.d.( passport, utility bill and a new thing PPS numbers). This means that the beneficial owner of the account is the primary account holder but has authorised me to be a signatory. It also means that she retains the benefit of having DIRT not deducted from her account - this cannot be the case if I was a joint account holder. In due course, I have an Enduring Power of Attorney and should we get to the stage that she is no longer capable of managing her affairs ( as opposed to simply not being physically very mobile) I will register the EPA and act as her attorney.
In the other case, the bank refused to accept a Power of Attorney to enable me to act on an aunts behalf and I had to get a medical certificate confirming that she was compos mentis and I can now access her funds.
I strongly suggest that an EPA be drawn up for OP's family member so that funds can be accessed in due course. The joint account is also a terrific idea - and even if it seems a bit of a faff , you have to anticipate what will happen down the line.
mf