What can I do to please Allah and mend the relationship with my uncle, who has disowned me and threatened my sister and her husband after her marriage, knowing that I am her guardian and did not consult him about her marriage due to his prior rejection of anything concerning us?

If the uncle mistreats and severs kinship ties, there is no blame upon the one who shows kindness and maintains kinship ties; he should continue to do good. As for the uncle's death threat, if he is serious, he should be reminded of Allah and the punishment for a murderer, and the marriage should be delayed if it hasn't taken place yet until things calm down. If the threat is merely to intimidate, it should be disregarded, and good treatment should continue.