After that, I explained Sylphy's situation and the fact that she has to hide her gender in public.
Well, as for whether or not I can tell people about this, Ariel gave me the right to do so based on my judgment, though my little sisters might be too young for this.
Since we are living together, they would find out about it sooner or later.
It's not wise to lose their trust by hiding things from them, and this way, getting their cooperation is easier.