Very much depends on his capacity to make decisions, and on his state of health, both physical and mental. If he has given Power of Attorney to his new wife, you are stuck. If he has not done this for her or for anyone else,and if he is capable of making a decision, ask him to give you these powers ( see the website of the Public Guardian)
If he is unable to understand or make decisions, you have to apply to become a Deputy in the Court of Protection, which is an absolutely ghastly and desperately expensive business, and can take 2 years.
My brother and I had to do this for our father, who was in a hopeless state of dementia, but did not accept his inability, and was extremely difficult.
We had to disclose to the Court of Protection the existence of any other close relatives , so you would have to mention your stepmother, and she would have to be included if she wanted to be included.
But try to avoid this, as the bureaucracy involved is positively Dickensian.