OK my Mum did not need a visa but if she did and was applying now.
She worked until retirement age, which in the UK is now 67, I do not know what the retirement age is in PI.
She had a State Pension and a Work Pension, my Dad did not like travelling but she did so was always off somewhere.
The big issue for her last trip was Travel Insurance, by then 2 weeks Travel Insurance was the same cost as the flight! Hopefully your MIL does not have any medical conditions.
They had a house they had lived in since 1959, like I said my Dad did not travel, and if the situation had come up a lot of reasons to go back, biggest one was Medical I guess.