You asked if the elders telling your brothers JW carers not to work with him might be illegal
It woukd be worth checking this out with citizens advice.
Here is what it says in the equality act 2010. Note also, that under the national care standards, as a service user he is entitled to basic human rights such as choice.
I would imagine his JW carers could face disciplinary action if they refuse to work with him as might be construed as discrimination against your brother simply because of his personal beliefs or views.