For an Adult Dependant application to succeed, the applicant has to:
Due to their age, illness, or disability require long-term personal care to perform everyday tasks e.g. washing, dressing, and cooking
Are unable, even with the practical and financial help of the sponsor, to obtain the required level of care in their specific home country because it is not available and/or there is nobody in that country who can reasonably provide it or because it is not affordable
Be adequately maintained, accommodated, and cared for by the sponsor without needing to access public funds. If the sponsor is a British Citizen or settled in the UK, they must sign a 5-year undertaking to that effect, at the entry clearance stage.