The house that the old lady was in was Anton Ego's home. If you look at the wooden chair in her living room, it has a parculiar symbol/design on its back, the same one in Ego's flashback when he ate the ratatouille. Since Remmy had been living in that house for so long, he must've stumbled upon a diary belonging to Ego or his Mum stating his favorite dishes as kid. Giving Rammy the reason why he chose Ratatouille dish in the first place.
