I can partially relate to your situation
I am adopted, and although I have known my real family on my Mothers side since I was born, approximately 15 years ago I decided I wanted to get in contact with my real Father. I know this isn't a sibling thing, but there are similarities.
This caused a lot of hurt in my family, but they were all very supportive, and knew that the day would come when this happened.
A few weeks later I met my real father, and we were nothing but equals. As it turned out, he had been waiting for that day too ever since I was born. We met a few times, but didn't really get on, and we have now lost contact again. I don't regret getting in contact, and I don't regret losing the contact either. I have satisfied my curiosity.
I can't say one way or another if you should allow the contact - only you can decide that, but as you are the one being contacted, you have the 'upper hand' so to speak.
I'm not sure if this has helped you in any way, but I wish you luck in whatever decision you make