Currently, a script solution is required to pre-select a sheet different from the first one.
function pageLoadHandler() {
    var spreadsheet = $find("<%= RadSpreadsheet1.ClientID %>").get_kendoWidget();
    var sheet = spreadsheet.sheetByName("Sheet1");
    spreadsheet.activeSheet(sheet);
        
    // Sys.Application.remove_load(pageLoadHandler);  
}
Sys.Application.add_load(pageLoadHandler);  
This script could be entirely loaded from the server-side following this article on how to properly register a script from the code-behind: