Contacting half sister

Hi, although I've discussed my situation with many of my friends, and my partner, none of them have any experience of my situation and I'm hoping to find a little advice/discussion on my situation. I traced my birth mother over fifteen years ago, and through NORCAP in the UK, an intermediary service I made contact, via three consecutive letters with no response. I also followed those up with a personal letter, asking at least an acknowledgement so I could be certain the letters actually reached her. These four letters were all unanswered. During my original tracing research I discovered she had got married and had two more children. I know their names and dates of birth. A couple of years ago I found them on Facebook and occasionally look at their profiles, I have never contacted them. I took a DNA test in October and have been linked to my half sister as a first cousin, I know she is my half sister as I applied for her birth certificate when I was searching for my birth mother. She has not logged onto Ancestry for over six months so presumably won't have seen this match. She lives half an hour away and hangs out in some of the same places as me. I'd really like to make contact but the words of that first message are alluding me. Do I tell her I'm her half sister? I feel like I have to, because I have to be honest up front. My partner says I have the right to make contact, even though my birth mum never responded. I don't know if my half sister even knows I exist so feel wary of possibly revealing my birth mothers secret? I've never participated in a forum of this kind, but I'm finding it difficult to make a decision. I feel like I'll regret it if I don't make contact but worry I'll cause possible problems if I do. Caroline