Caring for a loved one with a serious illness like acquired brain injury (ABI) or primary malignant brain tumors (PMBT) can be a challenging and burdensome experience, often affecting the well-being and quality of life of relatives. While family and friends can provide some support, this is not always enough to address the diverse needs of caregivers. Relatives often have different requirements for support, information, and involvement in the patient's care. Understanding these varying needs is key to ensuring both the patient and their caregivers are effectively supported during the illness journey. This study aims to develop and test an intervention that helps healthcare professionals better support relatives in their caregiving role. The intervention uses a dialogue tool, which is designed to help nurses assess and follow up on the differing needs of relatives for involvement in the care of patients with ABI and PMBT. The goal is to enhance the experience of both the patient and their caregivers throughout the illness process. The study will take place across seven units in two regions of Denmark, and it involves relatives of patients with ABI or PMBT. The main questions the study aims to answer are: * Can an intervention that involves relatives improve their role in the care of patients with ABI and PMBT? * Will this intervention help both patients and their relatives feel more supported and satisfied during the treatment process? Participants will receive support through an intervention that includes several key components: * Nurse training on how to use the dialogue tool to assess and address the varying needs of relatives for involvement. * A list created by relatives, for relatives, to share experiences and advice with one another. * Video materials explaining the background and purpose of the intervention to help relatives understand how it can benefit them. * Guidelines for documenting the involvement of relatives in the patient's electronic health record. By offering targeted support to relatives, this intervention aims to enhance their involvement in the treatment process, ensuring that both patients with ABI or PMBT and their families are better supported throughout the course of the illness.