As an attorney Latigo...Do you think OP or his wife have the legal right to challange the Stepfathers daughter? Based on what the OP has posted here.
Not sure what you mean by "the legal right to challenge" or in what setting, formal or informal.
But in addressing the issue as presented to us I think it proper to assume that due to some form of mental and/or physical impairment the subject person is incapable of making or communicating responsible decisions concerning his personal needs and care.
Under Florida law any resident of the state who is sui juris and is 18 years of age may be appointed guardian of a resident ward (F. S. Section 744. 309 (1)(a) Consanguinity is required between the applicant and the ward only if the applicant is a non resident of the state.
So it may be such that the daughter-in-law would be qualified to file a petition to determine incapacity and seek her letters of guardianship. And should it come to that, and even if the daughter also filed or objected to the petition on whatever grounds she might chose, the paramount consideration would not rest with their respective relationship to the proposed ward.
The preamble to the Florida guardianship statutes make it very clear in that it is the bests interests of the ward that must be considered. And personally I don't think the daughter could convince the court that removing him from his home and spouse would be in his best interests.
On the other hand I think the daughter-in-law, in the right formal setting, could very well successfully challenge the daughter's plans to remove him. Nor can we discount there being an ulterior motive on her part.