How should I respond?
Gypsyfish
Say hello, and that you're excited to find out about him and your nephew. Ask where they live. Tell him a little about you. Exchange a few messages, then see if there's any way you can meet in person.
Alex
Don't panic, and punch him in the face I'm just kidding, just act nicely and say Hi i guess I am ur half sister/brother. Guessing you would have a tough time
Pearl L
ask if you can nneet hinn sonnetinne
Marie And Alan
Why not meet up if you live near enough? Such relationships can be very rewarding.
real estate guy
If you are mad, the problem is with your father. This guy didn't do anything to you so you should have NO ill feelings against him. He basically is in the same boat as you. What do you want to do. You can do nothing and forget it. You can reply or call him and talk. It's up to you.