World seems to adher to strict Salic law when it comes to royal and aristocratic inheritance, especially so considering MC is the king and not prince-consort with Yuria as the sovereign queen. This whole thought wasn't even entertained during the whole civil war mess.
Gramp's heir apparent is probably closest male relative, if they have a dynastic system and not some weird electorship. Should Tetra birth a boy, he would then be the heir apparent. MC as the spouse of his kin also has a claim to the throne, once more supposing a dynastic system, but a rather weak one. Such a claim usually would require military intervention to enforce, atleast historically.