The blonde beauty ended her on/off relationship with actor Balthazar Getty earlier this year (09) and is moving on by getting close to the British DJ, real name George Barker.
The pair reportedly met at the Secret Garden Party in Cambridge, England last month (Jul09),and have been inseparable ever since.
Miller decided it was time for her family to meet her new love interest - and jetted him to Spanish party island Ibiza to join the festivities for her step-mother Kelly Hoppen's 50th birthday, according to Britain's Daily Mirror newspaper.
A source tells the publication, "Sienna and George are really quite besotted. They haven't been dating for long but decided to just go for it and get away for a sunshine break. Sienna and her step-sister Natasha had organised a surprise 50th birthday for Kelly, and Sienna invited George along too so they could meet informally over a glass of bubbly.
But Barker had competition for the Alfie star's attentions - fellow guest James Blunt was reportedly keen to get close to Miller.
The source adds, "James Blunt didn't realise Sienna was spoken for and seemed keen on getting to know her better - until George got wind of it (heard about it), and swooped in to rescue her.".