New Zealand Law
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If she is not an owner of the home, then she has only the right of any visitor, and you could ask her to leave. However if your husband is inviting her to stay that can cause a problem, because as an owner, he can permit her to live there. So these two rights cancel each other out. But your husband appears to be supporting her side and not you. He is wrong in saying she has a right to stay there because she is only a guest even if she is family.
Would you consider a family mediation to discuss this behaviour in a safe way with a neutral facilitator to help sort the problems? This may be the best way to move forward
You can give her a trespass notice, and then ask the police to remove her. It isnt her home-it belongs to you and your husband, and she is only there at your say so.