you'd have to progress through rinny's quest to the masters mansion at the third part of the graveyard (make sure you find pixie first)

First floor, entrance, go right, then upwards in a room right of the kitchen.

so once you are in the the graveyard (which you can get to by having Rinny follow you to the graveyards gate in case you arent already inside the graveyard) you need to navigate the underground crypt area by going down, up, right, then up the stairs. and then walk along the path until you reach another staircase. from that staircase go left, down, right. to avoid a game over after going right you need to have your mysticality up to level 4. once you done that and rinny has become trapped in the mirror then you need to either surrender to the Grave Hands enemy in a random encounter, check (and get captured by) the haunted tomb inside the crypt, or find Pixies lair in the graveyard (it is at the bottom of the screen near the staircase second down into the crypts. you have to randomly wander between the grey tombstones at the bottom). then she will meet you at the gate to the manor which is just past the area with the mirror.