Ah your mother seems like a wonderful person. I love reading stories of accepting family members. Good luck! (also I might be able to help with binder recommendations if you need some)
I just wanted to say that I think you're really brave.
My best friend since birth has been my younger cousin. We were born exactly a year and seven months apart, and I've always felt it my job to protect her. At 12, (I was 14 at the time) she came out to me. My love for her never changed- I've always been in awe of the strength that she has. Just after her 19th birthday, she came to me telling me that, after much consideration she was going to start her transition.
He is now 21. And just like you said, it's...different. I still love him every bit as much (truthfully, seeing the strength he has, I love him even more) than I ever did. I mourned the loss of Kristina, but I celebrated a birth of Kole- I know that that little girl is gone, she is a wonderful memory that I will always cherish. But with that figurative death, came a birth. He is now on his way to being the man he wants to be. I wasn't sure how to react initially, so I did the only thing I could think of when the time came- I gave him his very first testosterone shot. We have a long way to go, but it's a slow process (the surgeries to follow are a bit of an arduous task, which, frankly I am ready to be over), because seeing him in pain is the worst part but with every step, we're a little closer. It's almost a bittersweet feeling. I am so proud of him, and I am so proud of you.
I felt like every question I had was stupid, but he, much like you, welcomed them. Know that your acceptance for them means the world.
If you ever need to talk, please feel free, I'm always around. Good luck!