Sabriel would second this! And suggest that 'Think of your older brother' and 'Think of your younger sister', are clues to which locker it is- assuming the day of the week count starts at Sunday, that would mean lockers 42 and 24 should be searched, if Hortensia didn't have a locker reserved for her personal use.

And then move on to searching Hortensia's home if that doesn't pan out.

EDIT: After further discussion, Sabriel will also suggest searching lockers 31 and 13.