A power of attorney grants a person the authority to act on behalf of the grantor, and does not deprive the grantor the power to act in a manner that they would be authorized to, in lieu of a power of attorney. If A has power of attorney to act for B and B has power of attorney to act for C, A does not thereby gain power to act for C, nor does A take away B's authority to act for C. So power of attorney for your wife's father rests with his wife or sons. (An orderly way of arranging this is to state one person with authority, and successors, so let's assume that's what happened, and not that all three have equal and absolute power. But, coagents are possible, ni which case everybody has to agree).
There are a number of types of POA: they can be general, or restricted to certain uses (such as medical or financial transactions; or whatever the document specifies). We must assume that the POA in question is durable, otherwise the father's POA is ineffective because he is incapacitated. In Washington, for example, the law requires the words "This power of attorney shall not be affected by disability of the principal" or "This power of attorney shall become effective upon the disability of the principal", or similar words that clearly state the intent. Your mother-in-law would thus be able to act on behalf of the father, even if the brothers cannot or will not (assuming that all three were granted equal power); your wife could not (under any circumstances not involving the court).
A problem would arise if all of those with power of attorney were either unwilling or incapacitated. Supposing that the order of succession is Spouse, Son1, Son2, then Spouse has authority until her authority terminates with death, incapacity or resignation; and so on down the line. Subsequently, the court would judge what happens next, e.g. whether the daughter should be given power of attorney: power of attorney is not transferable (i.e. the last son could not pass the power on to someone not specified in the POA document).