The sister can probably contest the beneficiary designation, but this is fact dependent.
(I assume that the stepfather is dead; otherwise a suit would not be ripe because the stepfather could change the beneficiary designation before his death and a beneficiary designation that can be changed is not a property interest that is legally protected in the absence of an agreement not to change that designation such as a divorce decree. I also assume that the sister is also the daughter of the stepfather. This answer would be true in the United States and probably most other countries, but I can't speak to the law of all other countries.)
To contest a life insurance beneficiary designation the sister needs to have standing to bring the contest, which means that she she has suffered an injury to a legally protected interest if her theory of the case if correct.
This basically means that she would inherit some of the money if the beneficiary designation were declared to be invalid.
If the father had no will, or if he had a will leaving some fraction of his estate to each of his children, or if he had a will that was invalid for some reason (e.g. it violated the terms of a divorce decree and would have left her something if the divorce decree had been honored, or he was not competent to make his will when it was executed) then she would have standing to contest the beneficiary designation
Of course, she would also have to have some legal basis for the contest such as a lack of capacity or undue influence when the beneficiary designation was made by the father, or the existence of a divorce decree mandating a different beneficiary designation, or an allegation that the son killed the father and was disqualified from benefiting under a "slayer statute".
The sister could also contest the designation of the son if she was previously the beneficiary of those policies and alleges that the father's change of the beneficiary from the daughter-sister to the son was invalid for some reason (e.g. the stepfather lacked capacity or was under undue influence when he made the change of beneficiary).
But, if the father had a will that disinherited her (and anyone else, such as her minor children, which she had the authority to sue on behalf of), and if she did not also contest the validity of that will, then she would not have standing to challenge the beneficiary designation, because neither she, nor anyone she had authority to bring suit on behalf of, would receive any of the proceeds, even if the beneficiary designation was invalid.