I have one sister-in-law who's hated me for 30 years. She's notoriously rude (and sometimes mean) to me. I don't know why - I used to care.
Early on in our marriage, my husband saw this pattern of behavior with his "baby" sister and never talked to her about it. When she realized she couldn't get me upset (at least visibly), she ignored me. That was fine by me. But when my kids were born, she started being mean to them! She'd say mean, unkind, and hurtful things to them from the time they were quite little. At that point, I just stopped attending most of his families get togethers. Eventually, so did my husband. I didn't ask him to do this.
Siblings have to accept each other's spouses, or just get out of the way. Period. There's really nothing to "negotiate."