I’ve found some Ancestry hints to be helpful. OH has a 1/2 sister which he’s never met. Mum remarried and they apparently went to Australia. Ancestry came up with someone with the same christian names different surname. Decided to look into it and it was oh sister, still in England. She married under stepfathers surname. What clinched it was the children‘s birth registration had her mmn. Shame though as she died in 2009.