Welcome to the forums and congrats on trying to seek some guidance on this matter. This shows that you really care for your sibling.
I have a friend who recently came out as transgender. He is now happier to have all his friends know who he truly is. As a friend, I wanted to make him feel as comfortable as possible as I knew coming out is a very uncomfortable and vulnerable situation to be in. I am a gay teen and I recently came out, so I know the pain it can cause if the people around me do not accept me for who I am. Therefore, I knew I had to make sure that my friend felt as if he could confide in me no matter what.
I say this because, you are Madeleine's sister; you are one of your sibling's closest family member/friend. As a sister to Madeleine, it is almost your responsibility to be there for for them. So I say to just be kind and accepting of who your sibling is. They are amazing and still the same person they were before you saw that text.
And as uncut_gems stated, do not "out" your sibling to anyone. This is imperative, so your sibling can decide to come out if they wish, on their time and on their terms.
Also, everyone has a different view or opinion, and no view is incorrect or correct. However, sometimes there may be the right thing to do, regardless of opinion, which is to be accepting of everyone, no matter who they are.
Hope you can figure this out!