Will a person be held accountable for severing ties with two paternal half-brothers with whom he does not communicate and who do not communicate with him?

What you are doing with your two brothers is severing ties of kinship, which is one of the major sins, and its perpetrator is threatened with severance from Allah Almighty. Therefore, you must hasten to reconnect with your two brothers, as there is good for you in this in both this world and the Hereafter. Severing ties of kinship is a sin short of disbelief, and whoever dies persistent in it is under the will of Allah Almighty.