The power of the internet is truly astounding.
My sister-in-law always knew she is a child of her father's second marriage and understood that she had a half-sister (from the first marriage) who had possibly emigrated to New Zealand. She knew her father's full name and date of birth, and thought she knew his first wife's name, but that was it. As both her father and mother are dead and she is an only child, she had no way of finding any more details, but she felt she would like to make contact with her half-sister or her half-sister's children (if there were any).
So I logged on to Ancestry.co.uk and searched against the father's name. Birth details checked, and a marriage was shown, with the bride's second Christian name matching what sister-in-law thought was her name. As it is an unusual name this seemed a likely match. Search the birth records and I find there was a child born the year after the marriage. Could this be the half-sister?
And so to genesreunited.co.uk. Search against the first wife's name, and lo and behold! Two people have that person in their family trees. I send a message to each of them, more in hope than expectation, but within 24 hours I have a reply from one, confirming a distant relationship, saying that she knows a much closer relative, and giving that lady's email address (with permission, of course).
An email to the new person brings a reply the next day. She is the half-sister's daughter and is delighted to have found more family, so I have given her my sister-in-law's address (again, after getting permission) and leave them to take it from there.
Just two days to track down a missing half-sister. Pretty good, I think. And my sister-in-law is delighted.