Apart from anything else, you and your brother need to sort out how to manage her money in the medium to long term. If she’s still capable of giving consent, I’d suggest the two of you urgently arrange lasting power of attorney for her financial affairs. You and he seem to have a pretty good relationship, so you should communicate directly with one another about her, rather than relying on her passing messages between you.
As fast as the £15,000 goes, it rather depends on what it was for. Was she reimbursing you for costs incurred caring for you? Did she know you had a particular need for money? Again, I think you should be discussing this with your brother.