No. Your MIL can give your brother-in-law a new Power of Attorney, if she still has legal capacity, but you cannot appoint someone else in your place.
Will you no longer be involved in the decision-making processes for your MIL's care? Because that's the only time the POA for health care needs to be exercised. Assuming you and BIL generally see eye-to-eye on things there shouldn't be a problem.
Does your BIL want that authority? Does MIL want him to have it? It really is between the two of them. You can smooth things along by assuring them you will not be offended by a change, but you cannot make the change.
Will you no longer be involved in the decision-making processes for your MIL's care? Because that's the only time the POA for health care needs to be exercised. Assuming you and BIL generally see eye-to-eye on things there shouldn't be a problem.