Disabled brother

Wife is a non-EEA national and currently has retained PR from previous relationship and has now applied for BC (I am a British and Irish from birth).

Her brother has mental disability and their mother is his full time carer. The disability is such that doesn't require ongoing medical treatment etc but does prevent work/ meaningful communication etc.

"The plan" has always been that when the MIL is no longer able to look after him that my wife would take over the reigns, obviously this was before she emigrated but she still wants to fulfill that obligation and we are fortunate to be in the situation that financially we would be able to do so without needing any external assistance.

Looking on UK BA website (sometimes feels I should set it as my homepage) I am not really sure if this is going to be possible or if we need to plan on leaving the UK when the time comes?

He technically will not be financially reliant on us as he has a trust fund but is reliant for the purposes of care, the UK BA site only talks about those that are financially reliant.