Biological siblings share the same genetics, but it can be hard to know how to connect with them sometimes. Letters are a good approach; they give a person time to consider how he wants to respond. If you're trying to write a letter to biological siblings, think about your intentions before you put pen to paper. Clear intentions will help your siblings know how to respond to your letter. Write a positive, honest letter to connect with biological siblings.
Start with "Dear" and address your sibling by her first name, if you're comfortable doing so.
Explain who you are, if he doesn't already know. A sibling who may not remember you or be aware of you will need an explanation about your parentage and -- more importantly -- who you are as an adult.
Give a brief biography, focusing on your current life. It's OK, for example, to say you went to college, but don't start telling her stories about parties and professors. Save that for a dinner in the future. Tell her about your life at the moment you're writing the letter so she can connect to you.
Explain what you hope to get from the communication. If you want a medical history, say so. If you want to spend time with him, say so. Be up front and positive. Keep it simple. Don't ask him to spend two weeks with your family at an oceanside villa when a meeting over coffee will do.
Ask her about her life. Don't include a laundry list of questions, but express genuine interest in what she's doing and how her life has been until now.
Explain that you want him to contact you, if you do. Give him different ways to contact you -- email, phone and address.
Add a closing such as "Hope to hear from you" or "Have a nice day." Make it something positive and nonthreatening. Sign your name after the closing.