I never knew that a person could demand payments from their son/daughter in law...
It is crazy, but the way of thinking is that the parents paid to bring up the child, so there's now responsibility in reverse to maintain the parents if needs be. I've heard some real horror stories, including children being forced into paying to maintain a drunken parent for many years.
(wasn't me that told her to go hang :P)
Normally done pre marriage but can be done post I have been told If I do so will this exclude my income and assets.
You need to get a legal opinion on this, I'm afraid. The courts may see it as an artificial attempt to separate your income in order to avoid paying the alimony payments, so you'd be be best to get someone to tell you definitively as to how the courts would perceive it.
Yes, the court can force you to sell assets if you don't make the required payments. What would normally happen if you fail to pay is that the court would appoint a balliff, who would then seize your assets accordingly to pay the debts. However, if there has been limited contact with the mother prior to the case, it's possible to apply to the court that the parent had no interest in the child until it was seen that there was cash to be had.
So, the first step is to find a female lawyer with a reputation for being nasty. Is your wife an only child?
I'd also read this, point 4 is particularly relevant:
You don't have any legal obligation to your mother-in-law in the UK.
Even if they were married in the UK and lived in the UK, the mother in law could still sue in the Polish courts to get the alimony payments and to enforce the payment through European systems.