Is selling a hunting rifle to the questioner's brother considered "a betrayal of trust" that makes the questioner sinful, especially since his wife and daughter objected to the sale, and the questioner refused to sell it to someone else for a higher price out of respect for his brother's feelings?

As long as you have not concluded the sale with your brother, and it was merely a promise to sell, there is no blame upon you for retracting the sale due to the aforementioned excuse. We advise you to strive for reconciliation with your brother and do not sever ties with him, even if he severs ties with you, for the Prophet, peace and blessings be upon him, said: "The one who maintains ties of kinship is not the one who reciprocates, but the one who maintains ties of kinship is he who, when his kinship is severed, connects it."