My late Nana told me that my mother's boyfriend had a daughter but she died. This was about 4 years ago. In all the time that I did see my mother and her boyfriend a daughter was never ever mentioned. I knew he had a son as they chose not to have me live with them so they didn't have to pay maintenance for him.
I don't even know why I am bothered about knowing as I already know my mother is a liar and a manipulator.
you'd be able to search births deaths and marriages records but I'm not sure if you'd have access after just 4 years. Do you know any names or dates? I'm sure someone more knowledgable will be along soon