There is nothing to contest as long as he is alive. It may well be that he is doing what you describe merely to avoid having to explain himself or to be subjected to pressure. If he is in his right mind and feels that he prefers to leave her more than he leaves you, that is ihis privilege.
In order to successfully contest his will after his death, you would have to proove to the satisfaction of the probate court that he lacked testamentary capacity when the will was drawn.
Florida case law does say that you can not contest a will while someone is still alive. The courts don't want to waste time because by the time grandfather passes - all the money may be gone. If you believe he is incompetent - then you can consider a guardianship proceeding - but this should not be undertaken motivated only to protect some potential inheritance as there is some risk of attorney fees. Seek competent guardianship counsel to review this.
There is another method to challenge wills after death, other than incapacity, which is undue influence. Florida now states that if your sister is a trusted person for your grandfather, and she helps procure these will changes, then there is a presumption that she unduly influenced him. This can be independent of his incapacity. But, you should be aware that this behavior you are seeing can be common even if no one is twisting his arm. If you want to affect your relationship with your grandfather, then put the effort into it now while he is alive. This may or may not affect his at death gifting desires.