How large an army can you afford? Generally, the sibling who provides care is in the best position to influence any changes in a will. Integrity is what you are when no one is looking, and probate courts are full of sad tales of late changes in wills (and property changing title).
There is no proven way to prevent relatives (or neighbors) from inducing a change in a will or from transferring property except for you to be the person who provides care and attention to the elderly parent. If you cannot be there, you need to provide moral (if not also material) support to the people who are doing the daily care and hope they are grateful enough not to lock you out of your inheritance.