Maybe your family thinks it will make it worse or it's an issue between you and your sister that they don't want to get caught up into. Did you try asking your family for their point of view? Or why they are not prohibiting your sister at cussing at you? Tell them it is too overwhelming for you and that you need their help.

It's inappropriate to cuss, especially to her own family and to cuss at a child. It's verbal abuse and emotional. Is it possible to move?