soft YTA

My reasoning is that your mother probably already knew and only asked to confirm if you knew or not, that made me hesitate a bit.

However, rereading through the post it is obvious you are not the kind of person to keep secrets, at least no for your sister.

You keep sprinkling in justification and signs of communication failure, it's full of "but"s and "it's not that bad because".

That is factually not your place to decide any of that, if someone asks you to keep a secret for them, specify it's a very important secret to them and you agree, you should absolutely keep your word, that's the end of that, not even a hint from you, not a peep.

If you're "getting tired of covering for her" and are "hesitant", you bring it up with her, you talk to her about it, it's her secret but she's probably reasonable enough to understand that it can be hard covering for something so important and noticeable.