Hi Jeff. What a stressful time it must be for you both to have to take on the role of responsible adults for both your mum and your brother. And so soon after the loss of your dad. 2016 will be a time of big change for you, but it sounds as if you are getting on top of the necessary arrangements.
Does your mum currently have registered Power of Attorney for your brother? If there is an existing POA then you would need to apply to the Court of Protection (COP) to have this cancelled and to put a new authority in its place (UNLESS you or your sister are named in an existing POA as replacement Attorney/s). If your dad had POA for your brother this would have ceased to be valid when the Attorney died (UNLESS... as above). You would probably need a solicitor to guide you through this process, which might involve a court hearing; not just filling out forms.
If there is no registered POA for your brother then you have 2 options. If your brother has mental capacity then he will be able to grant you Lasting Power of Attorney for both Finance, and Health & Welfare (2 separate POAs). If he does not have mental capacity, then you would need to apply for Deputyship via the COP. This is where the COP authorises a person (or more than one person jointly) to look after the affairs of someone who does not have the mental capacity to make a LPA.
For finances you may consider that it isn't necessary - it depends on what his income is. If he only receives state benefits then you can apply direct to DWP to be an 'appointee' to enable you to manage his benefits and communicate with DWP about these, and any future benefit claims. If your brother has additional income, such as interest from investments, or on-going payments from a compensation award, then someone needs to be actively managing his money in his best interests.
For health and welfare (H&W) it would seem very sensible to have either POA or Deputyship, so that you can speak to medical and social welfare people as needed, and make decisions in relation to his treatment and care. It seems that the Court of Protection seldom grants H&W Deputyship in isolation, so if you are going to apply for finance Deputyship you need to ensure that you have included H&W in your application.
With regard to your mum, the same options apply as to your brother. She can only grant LPA if she has mental capacity. If she does not, then you have to go for Deputyship.
I recommend that you and your sister apply for joint and several powers and that you include the name of a replacement Attorney/Deputy (if there is someone suitable) to take over if something happens to the both of you. If there isn't anyone else suitable, then you could choose to name your solicitor as the replacement Attorney/Deputy.
[Edited to add: solicitors cannot be appointed with COP delegated powers for H&W, only finance.]
