I do think it’s worth exploring with them how much research they have done about the reality of living on the Isle of Wight. As others have said very different to a summer holiday.
In their 60s health care is going to be a major issue and sadly provision on the Isle of Wight isn’t great and they are having trouble recruiting GPs (more so than the mainland) The local NHS trust is amalgamating with one in Hampshire and many services in the future will be led from the mainland rather than the local hospital so this will likely involve travelling to the mainland. Not great when getting older and need for services will be greater.
Social services on the Island are stretched due to the demographics - more so than the mainland.
Have they ever lived somewhere which is a summer holiday destination ? Traffic / crowds etc are a nightmare during holiday times and local services are stretched, roads are busy, parking in the town is a nightmare.
Do they realise that many companies will not deliver to the Isle of Wight as it is so expensive transporting “stuff” on the ferry
Even with local resident discounts the ferry is expensive and during the peak holidays difficult to get a booking - particularly when one has broken down ( which happens more regularly then they might realise)
Ultimately it is their choice and sad that they didn’t let you know this was their intention before you moved to be closer to them, but I do wonder if they have realistically thought through and researched the practicalities of moving to an island.
Not an Island but my In laws suddenly announced a move to a UK holiday destination - many hours from their home and family, but after initial “it’s all fine and we will be ok” it transpired they hadn’t researched things like healthcare etc. and then realised actually it would be a mistake