Is a person considered an apostate (kafir) if their relationship with their siblings returns to its previous state after they swore, saying: "I shall disbelieve in Allah the day my relationship with them returns to what it was before"? And what are the implications of that?

A Muslim is not permitted to swear by other than Allah. And whoever swears by disbelief or that he is innocent of Islam does not commit an act of disbelief unless he harbored disbelief in his heart. The majority of scholars hold that this type of oath does not require expiation, rather repentance and seeking forgiveness are obligatory. Others are of the opinion that expiation for an oath is obligatory. The more cautious approach is for the man to seek forgiveness, repent, and offer expiation for an oath.