joleigh but the OP doesn't know herself yet.
She told her sister they would be finding out, not that they already know.
She wasn't rubbing her sisters face in anything.
She has good reasons for keeping the sex of the baby to herself. Her family "went into over-drive" and "tried to take over" her last pregnancy when they were told the sex of the baby. She doesn't want that this time.
She slipped up when she said they were going to find out, because they are already now making demands and behaving badly, but I don't think she told them because she wanted to rub their faces in anything. We've all said things we regret the moment the words have left our mouths.
To be fair to her, if this is how her family are already reacting when she doesn't know herself yet, I can take her point about the way they might behave if she does find out and shares the news.
Her sister wants to know the sex of the OP's baby so she can dictate that the names the babies are called are not remotely similar.
Her mother is already giving her the cold shoulder in favour of her sister, and the OP isn't surprised at that.
To me that sounds like there are issues with her mother already, she's hurt but not surprised by her mother's behaviour.
They sound like they will add considerable stress to a difficult pregnancy, and in that situation I doubt the OP wants to add to the drama by attention seeking or rubbing their faces in anything.
She spoke without thinking when she said they would be finding out, and is clearly regretting it now. But that's a different thing to attention seeking or vague booking, and it's certainly not rubbing their faces in anything.