This doesn't work in my opinion. Since English is an unfortunate language with no gender neutral singular third person pronoun, I also use "they" occasionally in written text when the gender of a person or character is unknown. But can you actually imagine Sakura using it like that in a conversation like that, to refer to a person present right there and then? Obviously in the original Japanese it wasn't any issue, but in English it looks unnatural. As far as I know, it's meant to be used when the gender is unknown, not... I don't really know why it was used here. It's not like Sakura wouldn't know Amane's gender (whether it's the biological or desired) or couldn't choose a pronoun that wouldn't sound strange in the big sister's ears, to avoid trouble. If you imagine this conversation actually happened in English, wouldn't the big sister be slighty confused by that "they/them"?
Still, I'm naturally extremely thankful for the translation, so pardon my criticism, should a member of the scanlation team visit this comment section.