But he was nominated heir without a blood claim, so he can serve as a proof that Tsar could still appoint heir before Peter. Sophia had two younger brothers and no one appointed her as a heir, so no surprise that she didn't press her own claim as a Tsaritsa.
Peter issued a law according to which Russian ruler is completely free to appoint anybody as a successor. Such a law did not exist before and tradition assumed succession by the closest
male relative. Culture allowing woman to succeed was not there, yet, and you can’t ignore the cultural framework of a society. By the time of Peter’s death Russia was already an empire with its own laws and pretense to be “European” so the framework was seriously different.
Argument about Sophia having the brothers is not working: by the time of Peter’s death there was his grandson, future Peter II. Peter did not leave any will (and did not nominate her) and his law (rather typically) left a field open to the discussion. Supporters of Catherine had been arguing that Peter is young while Catherine is mature and already has experience in the state affairs (actually, she did not) and that Peter could be her heir. The dispute had been won by the Guards who promised to break the legs to anybody objecting her candidacy.
Now, by the time of Feodor’s death inheritance by the woman was
culturally impossible. There were
two coups: 1st failed conducted by Naryshkin clique declared the younger brother as a Tsar (claiming older brother’s bad health as an excuse) and 2nd, conducted by Miloslavsky clan, resulted in declaring
two co-Tsars with Sophia as a regent.
Culturally, she could not get more even with the military force (Streltsy) beh