Half-sib made contact

Hello internet strangers! I am hoping to reach out to your collective experience and ask for some insight into a situation that I am currently trying to navigate. Any insight/advice/input from this community would be tremendously appreciated. At the start of the year my younger half-sibling had tracked down and connected with my sibling through social media. Understandably this person wants to know more about us and their biological mother and why they were given up for adoption. Since this connection was made a few weeks ago, I’ve been in a difficult spot emotionally and do not know what the best way to respond is. There is (from my perspective) a significant mitigating factor that has been greatly weighing on me. The best way to explain this is to provide some additional context: Our father passed away when my sibling and I were quite young. A number of years later our mom was engaged to be married, but unfortunately she experienced the trauma of being sexually assaulted by her fiancée. Although pregnant, she called off the engagement. She opted to not terminate the pregnancy, but worked on finding a family to adopt the child. At the time, my sibling and I were quite young and knew none of this: only being clued in about a month before the scheduled delivery. I remember being incredibly disappointed when I found out that my half-sibling was to be given up for adoption, but it happened in a country and at a time when the opinion of children did not weigh heavily on any parental decision-making. Complicating matters is that shortly after our half-sibling’s birth in what I presume was an attempt to try and deal with the trauma she experienced, we emigrated from our home country. Unfortunately this posed significant challenges – financial and otherwise, that has taken nearly two decades to correct. I mention this because I have at times thought of my half-sibling and was thankful for the fact that they had a loving home with a pair of parents and did not have to struggle to establish themselves without a support network. Fast-forward to 2019 and the connection is made through social media. From my understanding, my half-sibling only recently came to the knowledge of their status quite accidentally. Somehow they have been able to track down their biological father in their quest for answers, but they were not given the truth. The fact that our half-sibling has made contact has not been made known to our mother. I made the call to not let her know as she has been fighting to get through a terrible divorce for over two years and is in an incredibly emotionally vulnerable state. From my perspective I feel that this would be too much for her to handle right now. How do I deal with this? I cannot imagine what someone must go through when they realize they are adopted, especially if this has not been apparent to them from a young age. My worry is that it is inevitable that the question of “Why was I given up?” to come up. Given the context outlined above, how can we have an honest conversation about this? In my mind, finding out that you are the product of an assault would be absolutely world-shattering. In discussion with my sibling, they are more than content to gloss over this fact and not divulge the full details. To me, this is disingenuous, and I feel that if I am to start a relationship with my half-sibling, I do not want it built on a half-truth. Again, any insight that you can relay will be much appreciated as I try to navigate through this.