This applies for any two muslims, but in the case of blood relatives this can lead to severing the ties of kinship and it is haraam, you have to seek reconciliation and the one who says salam first will be rewarded more. The prayer is not affected by this.

I shall be grateful if you kindly let me know if it is true that if a Muslim does not talk to his brother for three days or more his prayers are not accepted and his sins are not forgiven despite his repeated requests to Allah for forgiveness and even in the month of Ramadan. If yes then does it apply to real brothers and sisters or to any Muslim relative?