Is his wife using him?

The fact that she is pissed off he has booked to go and visit her and his children at Christmas instead of staying here and continuing to work over the holidays to send her more cash instead....pretty much answers your question.

If this actually does all go to plan and she isn't using him to build her a house before she dumps him, what exactly is he going to do over there, does he know? has he thought any of this through? because it sounds like he will be expected to be the one bringing in all the money.

They have children (plural) that ship has long since sailed. He has to move to Poland now, to be with his children.

I agree that if she genuinely doesn't want him to come over for Christmas that's a worry, but is that what's happening, or is it that they'd worked out a timeline for completing the house and moving in together permanently as a family, and missing out on the lucrative work (a few thousand € perhaps) over the holidays plus paying to travel at peak time (maybe another 1000€ if hire cars or more expensive than usual transport to the rural location on a bank holiday and so on are involved at peak time) will mean not meeting the construction bills they've committed to on schedule?

It would be interesting to hear the sister in law's side of the story as her in- laws also sound as though they don't like her and try to damage her relationship with her husband and interfere in their family finances.

The fact that this isn't a new relationship but one with at least two children means the brother and his wife are their own nuclear family and his mum and sister need to respect that he is a father to the children and that has to be his top priority.

The fact she doesn't want him to see their children over Christmas is hugely worrying. Most mothers want a family Christmas, it seems to be genetically wired into us. How often does he see his children?

So he will have to learn the language, won't he? It's such a cliche - English people moving around the world expecting everyone to speak English. There is certainly no shortage of opportunities to learn the language while still living in the UK. I bet his wife learned English while living here. Why can't he do the same?
Don't know about the house being in her name but I imagine that Brexit changed quite a few things and it may well be an easier option. I am forced to sell my flat due to changes in tax brackets due to Brexit.
Your brother is an adult. They made a decision as a family to move somewhere where his savings buys them a much better quality of life. If he is happy with this plan, I wouldn't say a thing. The fact that you made him cut up his credit cards says quite a lot about your level of involvement in his personal life and finances and, of I were his wife, I wouldn't be happy about you meddling either.

I think the OP is right to be concerned in this situation and also that its not intruding on her brother's personal life. Some people are more vulnerable than others to bring used.
This is where those of us with functional family relationships have the advantage as we look out for each other.

If that was my brother I would take the piss out of him and call him what he is...... "A total mug". It is your duty as his sister to do the same.

OL if he is a high earner then surely a house in Poland will be very quickly paid off. As others have said, £100k will build a decent house. In the random location you mention it will be even cheaper. Even if it's a mansion it will be less than £200k.
Where is the rest of his money?

Are you saying if a close relative or friend was being scammed you wouldn't get involved because you would rather mind your own business?

Wow

A mother in law who unilaterally decides that a high earner doesn't need credit cards doesn't sound great IMO. How does he pay to refuel his car or for holiday flights? Both much better on a credit card paid off in full each month.

In that scenario, I can see the attraction of settling in a different country.

I think the fact she unhappy he's booked to go and visit her and his children is not a good sign.

But, what is the plan?
Surely if they had decided to move the children over there and he will move later on, then they have a plan as how to support themselves financially.
Like, will she work? Is it a touristic area where he can earn working in English speaking jobs? Is the house a source of income, like a chateau to be rented?
I would hate to leave my DH behind and take the kids. They will miss so much on bonding time with him. I would be in a hurry to finish it too, but I do not imagine I would get angry about him taking a break for Christmas.

I remember when I started being with my English husband 16 years ago and how much his mother and sister called me 'gold digger' and now I'm in deep dark hole with this man. Working my socks off everyday and him living like man of leisure.

You can't swap genders sexes. The world is different for men and women.

We found out he also has credit cards we told him he doesn't need them as he is a high earner but she was not happy that we encouraged him to pay them off and cut them up.

That was not your business to do. Using credit responsibly is actually good for your credit rating.

You seem very involved in your brothers life and finance's. Do you not trust him to make his own decisions?

Imagine the outrage if the genders were swapped

I don't know, to be honest! He might get an online job in English, or (as his family think) come home, find work with WFH built in, and commute. He really isn't making progress with Polish. It can work, though: my brother married a Spanish woman, and moved to her home city, and has learnt Spanish fairly easily (but not Catalan!). However, he's an English teacher, and has been able to work throughout. His daughters are (of course!!) trilingual in English, Spanish, and Catalan. And Spanish is relatively easy.

I moved with my husband to a rural part of his country of origin when I didn't speak the language (EU pre Brexit so not especially high risk). I'm generally rubbish at languages too but despite being heavily pregnant when we moved I knew had to learn the language, so I did. Not perfectly and almost 20 years later I still have a strong foreign accent and make some grammatical errors, but well enough to have done another degree here in the local language in order to retrain in an in demand profession and work full time in the local language.

It's a huge emotional rollercoaster for the first five to eight years and you have to be determined/ big headed/ stubborn 😜 but it is most certainly possible to learn a language as an adult despite being "bad at languages" when you haven't absolutely had to learn them and haven't been fully immersed.

Not saying that the scenario described is perfect but we cannot tell our capable adult siblings how to live their lives, and it's surely better he moves over and gives it his all than lose his children.

I'm living 20 years in UK so not the best about advice will be good to speak to solicitor Polish one. Also what I can see the law in Poland is more on women side also family help etc. if the house is 200.000 k will be a mansion.

I'm better looking younger Polish with abusive English husband don't be so predictable.... I will also go out with 50 k after divorce and nowhere to go.

I'm Polish tell him to protect himself if something goes wrong he will trapped also Polish families are very strong, they will support her all the way. He needs to start learning Polish ASAP. You can build amazing house in Poland for 100.000 K in some areas.

I would feel really concerned about this OP. I'm not sure what you can do about it though, other than mentioning your worries to him every now and then.