Need help... Long lost sibling

Alena

So long weird story. A year after I was born my father met a woman in Korea while he was stationed there and they had a baby she's now 23 and I'm 24. Apparently her birthday is a year and 2 days after mine. They came back to America and they didn't want to keep her so my father let his father adopt her. I live in NY she lives in Texas. So my grandfather who I've only spoke to like 4 times in my life is very strict about her knowing nothing. The last time I spoke to him a few years ago he didn't want her to know she was adopted or know with any of the family in fear we might tell her. She thinks our father is her brother. Anyway... Skipping to now, yesterday I found her on fastpeoplesearch and I was so happy to find her I have her address and phone number. I can't find her on any social media. I found out her name is really Alexandria but everyone I heard of her from says her name is Krystal which is strange because that's her middle name. Now I want to contact her so badly but me and her "dad" left off on a bad note last time we spoke and I'm not close to my father or his side at all. The problem is I don't know if he eventually told her and she now knows she's adopted, judging by how mentally incorrect my grandfather is in the head I'm guessing he didn't tell her. So I'm in such a dilemma, I want to contact her so badly but I can't call since her dad knows my area code. So I decided to write a letter. I would love to write her and tell her all about myself, my husband, and my kids (her niece and nephew) but I just don't know if it would be wrong to ruin what she has with her family (mom and dad). I'm afraid she's gonna be so hurt and something bad might happen between them because they have been lying to her for 23 years. A part of me wants her to have me in her life just as I want her in my life but I just feel bad about the whole situation. I don't know what I should do. Any suggestions? And also if I do write her, what should I write? I'm so at a loss of words to put on a paper. I wish I could just find her on social media or something. My mom thinks I should just tell her I'm a relative and ease into it but I'm nervous she will be upset with me and want nothing to do with me for lying to her, until I do tell her. Please help...