Challenging POA

First off, are you talking about a "current-style" LPoA? Or is it an older EPoA?

What stage is the PoA at?
Is it already registered, or going through the process of being registered?
There's a process to follow to object during registration, but afterwards it's a lot harder.
www.gov.uk/object-registration-power-attorney

What's the issue? Is it with the attorney's handling of affairs? Or because you don't think the right people are being chosen?
There's a process to follow to register concerns about handling - www.gov.uk/report-concern-about-attorney-deputy-guardian

Does the person still have capacity? If so, they can end the PoA themselves.
www.gov.uk/power-of-attorney/end

This sounds very difficult. Maybe it helps a little to know that this situation is not uncommon. Mine was bit different because the POA was not operational - it meant the difficult conversations happened when the donor was still able to modify terms and I could have the conversations with them rather than a third party. 

There are specialists (solicitors) who can give you advice as to whether there is a case or not. If you think things are badly wrong, I would recommend speaking to one because it will force you to gather concrete evidence and basically establish whether there is a legal case. If there is, you can use this as leverage for mediation, informally or otherwise, and hopefully that will jolt some common sense into the situation without going through the legal system. 

Is this a situation of coercive control?
The pattern of behaviour you describe between your relative and his wife suggests that it might be, and coercive control is a form of domestic abuse and a criminal offence, POA notwithstanding.

So likely doesn't have the capacity to end the PoA himself - or, importantly, to set up another one...

If this PoA was closed down, then any and every decision regarding your relative would have to be taken through the Court of Protection - and that's very slow, very expensive. Be careful what you wish for, you might get it.

That doesn't come under the PoA anyway.

As his wife, she has more right to restrict visits than you do to insist on them.

Likewise with that.

I suspect that they're actually their assets, marital joint assets.
If he had capacity, and they divorced, would they be viewed as marital assets in the financial settlement...?

And, again, that's not the PoA's business...

When he had capacity. Presumably, it's an LPoA that has been fully registered.
Anybody can check whether there's a registered LPoA in place, and who the attorneys are - www.gov.uk/view-lasting-power-of-attorney

It would be illegal for them to do anything bar tell you the very basics anyway, anything more would be a GDPR breach.

To all intents and purposes here, you are just Joe Public. Family trees, in and of themselves, carry no weight legally.

The job of an attorney is to act in the person's best interests - in finance and property, or in health and welfare. To make the decisions they believe the person would make themselves if they were capable.
The PoA is no more than the legal mechanism by which banks, solicitors, doctors, hospitals recognise the attorney's right to deal with them on the person's behalf.
Given that we're talking about a married couple, it's entirely possible a PoA isn't even needed to manage those medical or financial decisions. The wife will be next of kin as far as medics are concerned and, if the assets are joint ones, then they're as much the wife's as your relative's...

If there's no accusation that the attorney is going against your relative's best interests (which are not necessarily the same as ensuring maximum longevity) in their dealings with medical professionals, or is misusing the person's sole assets for their own purposes, then there's nothing they're actually doing wrong under the PoA.

I'm sorry you're in this situation, but I can't see from what you've said so far that you've got a legitimate objection - or that one would make any difference.