Your sister is out of line getting mad at you about this situation.

You're supposed to know what your mother is up to and act on it, but she's not supposed to know what her 8 and 10 year olds are up to? Your mum was supposed to magically know she didn't know about the accounts? What??

Um, no.

Hugs. She's upset / angry / guilty and lashing out.

I hope she calms down and backs off and (heaven forbid) apologises to you.

This is about her. Not you. You've done nothing wrong.

I hope you are able to enjoy your holiday.