Will I be sinful before Allah if I boycott my sister who repays my kindness with ill-treatment, especially after her husband threatened me with the police if I contact them again, knowing that she also boycotts the rest of my siblings?

It is advised to continue maintaining kinship ties and to be kind to relatives, even if they mistreat you or cut you off, based on the saying of the Prophet, peace and blessings be upon him: "If you are as you say, then it is as if you are feeding them hot ashes, and there will always be with you from Allah a supporter against them as long as you continue to do that." By maintaining kinship ties, a Muslim attains reward and avoids the sin of severance, for Allah Almighty says: "So would you perhaps, if you assumed authority, cause corruption in the land and sever your ties of kinship? Those are the ones whom Allah has cursed, so He made them deaf and blinded their vision."