Does the reward for patiently enduring the sister-in-law diminish due to complaining, and what is the difference between forgiveness and the loss of rights? Is the inability to forgive her considered a sin?

There is no sin upon you for speaking with your husband about what you suffered from his sister, and it does not diminish the reward for your patience with her, as long as your conversation was not intended to incite discord between them. However, it is preferable not to complain, and forgiveness does not diminish the reward of the patient person; rather, it increases it, as Allah Almighty says: "But whoever is patient and forgives, indeed that is from the matters of firm resolve," and His saying: "So whoever pardons and makes reconciliation - his reward is due from Allah." The matter of love and hatred is an affair of the heart, and what is required is soundness of the heart, free from malice, envy, and ill-suspicion towards Muslims.