Is it obligatory for a girl who committed adultery with her sister's husband to seek her sister's forgiveness and inform her of her act upon repentance, or does Allah's concealment of her sin and the prevention of breaking up homes preclude that?

We have not found any statements from scholars indicating that it is obligatory for the woman to seek forgiveness from her sister in this case. What appears evident is that sincere repentance between her and her Lord is sufficient, by abandoning the sin, regretting what has passed, and resolving not to return to it. She must avoid the non-mahram man and keep away from him; she should not be alone with him, nor should she remove her hijab in front of him. This is because leniency in the preliminaries of zina leads to it. Allah Almighty says: "And do not approach unlawful sexual intercourse. Indeed, it is ever an indecency and an evil way." (17:32) The prohibition of approaching is more emphatic than the prohibition of the act itself, as it encompasses all preliminaries and incentives.