Aim: To explore the experience and the preparedness of family carers in their caregiving role as best interest decision-makers of a relative living with advanced dementia. Background: The prevalence of dementia is a global issue. The role of being a carer of a relative living with dementia does not necessarily lessen once they are admitted to a nursing home. Best interest decision-making including end-of-life care decisions need to be made and reaching these choices can be challenging. The preparedness of family carers in this role needs greater understanding. Design: Descriptive qualitative study. Methods: During 2015 twenty semi-structured interviews were conducted of family carers of nursing home residents living with advanced dementia, then analysed using Braun and Clarke's thematic analysis. Results: Three themes were identified: (1) Caring for someone living with dementia. The impact on the carer's holistic well-being and their experience of being a best interest decision-maker; (2) Accessing support. The influential nature of formal and informal networks; (3) Perceived knowledge and understanding of the dementia trajectory of carers and nursing staff. Conclusion: The experiences and preparedness of informal carers is a reflection of their personal response, but the distress experienced highlights the significant need of adequate support availability and of enhancing nursing staffs' dementia expertise to maximize their role in facilitating best interest decision-making. This has significant implications for nursing practice and for service user and nursing staff education. Considering the global impact of dementia, our findings have international relevance to similar nursing homes across the world.