Gwen Stefani's household! The singer's eldest child, Kingston Rossdale, is celebrating his 15th birthday – and his half-sister Daisy Lowe was among the first to mark the special occasion.British model Daisy took to Instagram on Wednesday to share a very rare photo of the pair together and wrote: "Happiest birthday dearest @kingsrossdale_ what an incredible 15 year old you have grown up to be...MORE: Gwen Stefani seen with baby bump as son Kingston shares sweet Mother's Day message WATCH: Gwen