Hello, than you for your patience.
In response to (a) your sister can provide that her husband can remain in the house if she goes into residential care. This would not be in her will as that would not take effect until after she passed away, but she could make it clear that they were her intentions. Her husband is very likely to have matrimonial home rights which would allow him to stay in the property, as it is the marital home. The value of the home is likely to be taken into consideration when deciding if there is a charge to pay for any care. Such charge could possibly be deferred, i.e. not paid until the house came to be sold.
(b) I think it would be unreasonable and inappropriate for any council to expect your brother in law to pay rent to stay in the house.
(c) if your sister pre-deceases you brother in law, she could grant a life interest t him in the house, this would mean that he could stay in the house for as long as he wished. when he passed away or left then the house could be sold.
Generally it is the executors divide the estate between the beneficiaries. An attorney may have he power to sell goods for example to pay for care, but generally dealing with the estate falls to the executors.
If this answers your question please could you take a few moments to rate this answer. if you would like more information please click reply. Alex.