When I first came out to my immediate family, my sister was by far the least accepting of them all. I told her I was trans and she replied simply with "No you're not." And walked out. She refused to ever talk with me about it. For the longest time, she refused to acknowledge me for who I am. Granted, I haven't asked my family to call me by my preferred name or pronouns yet (simply because I'm still going to an all girls' school and won't transition until graduation). But recently, she's seemed like she finally gets it.
I've been wanting to get some ties for my outfits and she's joined me in pestering my mom about going to get me some- she told me she likes ties but "can't wear them", implying it was because she was a girl.
Then today, she was working on an exam for a class and needed someone to be a voice actor for a Roman Emperor. Instead of asking someone like my step-dad, who actually sounds like a man, she asked me to do it. Since I'm not on T yet or anything, I was really excited!
I'm so glad she's finally getting used to me being a guy

I wouldn't be surprised if when I switch to my preferred name and pronouns she goes back to how she was before, but this gives me hope she'll accept me, if not immediately, then eventually