My dad lives in FL and his wife died and left him everything in her Will. Do his stepkids have any right to a house still in her name? Probate?

If everything is left to him in the Will, once the court appoints an executor, the executor must follow the tems of the Will, including filing a distribution deed transferring ownership to the beneficiary under the Will.  Many spouses leave everything to each other.  The children may well be disappointed.  Perhaps your father would consider giving them a few items of sentimenatl value, copies of photos, etc. with which to remember their motherr.