I lived in Spain for 20 years. On the Med, La dolce Vita etc.....BUT both of us were working long hours and with regular travel across the country.
It very very quickly became clear that we could have guests staying with us 52 weeks of the year. My ex came from a huge/enormous family with, l think, nine sets of cousins etc. The only way to save the situation was to say politely that we would put them in a local Airbnb and that we would be able meet up most evenings around 8pm. In almost every single case it worked well. We explained that normally they would not need a car but a friend had an agency so they could expect to pay a maximum of 100 euros a week.
Sometimes we paid, often (especially the younger nieces) they volunteered (and even brought friends!) l have to say the arrangement worked well. We met up most evenings and they spent the days on the beach, in bed or sightseeing. We managed to work, they had fun. Nobody was embarrassed by their failings in English, French, Spanish, Swedish etc. Win-win.
I am absolutely sure that your MiL will be much happier in a local BnB. Money? Pay, if necessary, it will be much cheaper than a divorce.
Good luck. Tricky decision