How to deal with crazy family? ?

We've had nothing but issues with my husband's sister. For years she used family and her parents raised her son. She is so immature and acts like a teenager. At 38 she got pregnant again and ended up living off of her parents in their basement. Did I mention her mother was fighting for her life with cancer at this time too? So finally she moves away after realizing her parents have issues and can't do for her anymore. We figured we were free of her. No!! She starts sending nasty texts to my husband. Is there an app or something for an iPhone to block her?