I relate. Similar dynamic in my family. My father is also an alcoholic (now sober). My siblings drink, and we used to drink together quite a lot, as well as drink quite a lot together. My elder sister cut down some years ago, but another sister has not. Elder sister is supportive of my quitting, other sister does not quite get it. Like you, I tried explaining it to her. Initially, she seemed more interested in finding out how much I did drink, as a barometer for her own drinking, I felt. We don't live in the same country so it sometimes slips her mind I've stopped. Once we were relaxing together, and she seemed put out upon realising I wasn't drinking because it meant "she couldn't drink with me anymore". Which of course is not true - she can drink all she wants around me. We are due to meet up on a short holiday which involves wine country. Again there was a slightly pouty moment when she realised I wouldn't be imbibing. "Oh, but you won't be drinking." As if it would spoil her good time.
I don't know what it is - is it that our sobering up is too much of a mirror to our siblings' own drinking, as others have suggested? Or is it, as I believe, also having to face up to the fact that the ghastly form of alcoholism that blighted our childhoods is still not completely gone.
And like you, I'm fairly upbeat about my sobriety in general, but this lack of understanding from my sister rankles.
I completely understand on a rational level that it doesn't matter - I don't even see her that often. But the issue is there. I will ignore it, like you are choosing to. But I have a feeling it will show up again in the future in some form.