Agree with the above.
What does the Will say specifically? Does it say that the house is to be sold and the proceeds divided between the kids? Or does it just say that it is left to all the kids, and the executor gets to decide what happens to it...use it as an investment property, sell it, let another family member live in it etc etc. If it is the former, then your wife has to sell it, and distribute the proceeds accordingly. She has no choice or wiggle room. She has a legal obligation do carry out the precise wishes of her mother if they were so stated. If your mother in law wanted family to keep on living in the house after she was gone, she would/should have made arrangements for it in her Will. If she didn't, your nephew is out of luck.
What your wife wants to do, what you want her to do, what your nephew wants to happen, what would be most convenient for you all, what you think makes financial sense is all irrelevant. You should not let yourself get so bogged down in family dynamics that you all lose sight of the fact that your wife has a legal obligation to carry out her mothers wishes to the letter. If she does not, the sister (that is pushing to sell now) could come after her for any losses that she sustained while your wife delayed selling the house. For example, if the house sells now for 500,0000 euros, each sibling gets 100,000. If property prices keep falling & it sells in a couple of years for just 400,000, each sibling only gets 80,000. The sibling (and the others) could come after your wife for the 20,000 that they missed out on if they sold now. Or be owed interest on it. Something like that. There are tons of threads here talking about dealing with the sale of a family home, and conflict between family members as to what should happen to it. I am pretty sure I have seen that additional aspect of it mentioned more than once.
If all 5 of the siblings were in agreement as to what to do, I suppose you could wing it with renting to your the nephew, cross your fingers and see what happens. There is no one to stop you doing that. But as one sibling is already not in agreement with that happening (with possibly more to follow) you are already on a slippery slope towards a big family bust up with all the rows and bitterness that that entails. You can avoid all that conflict, and its additional stress by doing what the Will says. Sell the house and divide the proceeds accordingly.