Found out I have a sister from my mum side. Think she was born in the 50 I don't have a name or dob. Only know she went in to care my mother was in the knebworth. I have a brother who is 53 I think so she would be older than that.

You don't have enough to go on to search online, so start with your brother and any relatives of your mother - her nieces and nephews and any possibly living siblings. If you know where she lived at the time, you could go there and ask around the library, churches, and newspapers.

Why are you searching?

She may not even know that she is adopted!

If your mother is still alive and is not initiating the search herself, through valid reunion websites, I would let this go.