Are they in the UK ?
My partner recently found his sister who he hadn't talked with for 7 years. She was in the states - there are search agencies who can locate people as it is much easier to search information. Don't know if there are british versions ?
DP has now contacted both sisters, met 4 nieces who he didn't know existed and a couple of nights ago spoke to his mum for the first time in 25 years. For him it has been a positive experience.
Good luck.