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It will make a stronger visa application if your brother and his family can demonstrate that they meet the visa requirements without the need for a sponsor. Sponsored applications by their very nature are weaker. If they are going to stay with you they can say so in the application (and provide confirmation of this from you) but I would avoid complicating the application process with a sponsorship unless they need your financial support to meet travel and subsistence costs during the trip. UKVI does not require confirmed flights / accommodation reservations as part of the application process. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/614897/UK_Visa_FAQ_South_Africa_PPA_May_2017.pdf. Proving general financial standing and demonstrating a credible intention to return to home country at the end of the proposed visit is key.