I would start by calling the
Support Office to get guidance on your situation and also contact your bank to determine what type of joint account it is. There are a lot of peculiarities around joint ownership and whether it was set up as joint tenancy or tenant in common. If you have funded 50% of the account, then it should be fairly clear that only 50% belongs to your mother.
If the joint account was fully funded by you mother, then the type of account is very important as to what you own.
In the greater scheme of things, I would err on the side of caution and disclose all information. I would rather include the full amount and have it included in a successful application rather than have the application rejected for inaccurate disclosure of assets