Keeping ties of kinship with wrongful relatives

Q: I have two sisters-in-law, one of whom we have been boycotting for three years because of her hatred of us and her backbiting us in front of her husband (my brother). She is my cousin and lives in another city where we visit her only on occasions. However, after we severed ties with her, we never met each other, to the extent that when we visit the city where she and my brother live we do not visit them; rather we meet our brother at our third brother's house. Further complicating the situation is the fact that she has no respect for our parents, and my brother agrees with everything she says. We have forgiven her many times hoping to start anew with her, but her negative behavior continues.Are we considered sinners for severing ties with her and ignoring her? Please advise. We are so confused about this issue, and we do not want to sever ties with her based on our own wishes; it is her behavior that creates problems for us. She also caused an argument between my mother and my other brother's wife and when confronted about it, she denied it; although my brother's wife verified it in front of her. We severed ties with the other one (Part No. 25; Page No. 322) a year and a half ago because of a trivial problem with my father, and he died while still angry with her. Since this problem occurred, she severed ties with all of us, although she lived with us in the same house. A while later, she began starting fights again and abused me, my brothers, my sisters, my father and my mother. When we returned her abuse, she insulted our honor and called us bastards. We did not respond, and we never talked to her after she said this awful word. Allah knows she is a liar. It is a pity that she is our relative. She also abuses us in front of my brother, until he has begun hating us. Now she is trying to reconcile with us, so that she can mess things up again. Her husband, my brother, is married to another woman at the same time, and our relationship with his second wife is great. She severed ties with his first wife in an attempt to keep her away from us. Now she is trying to reconcile with us so that she can ruin this relationship.Are we considered sinful if we do not forgive her, given that we have reconciled many times and then had bigger fights, especially after what she did?