While she'd also go on to convince herself of the falsehood that despite being Lizzie's classmates in the strip, they were all the time in the evil and wrong Catholic school of learning to be jerks, it's somewhat obvious that they somehow became invisible to Elly at some point in the year 1990. Since we're looking at the Pattersons' world mostly through her eyes, the fact that she could no longer care to remember what they were doing tells us that at some point, they'd become dead to her. This seems to have taken place about the same time that Anne and Steve realized that Christopher was old enough to baby-sit so is readily explicable. Since the Pattersons no longer needed to be part of their lives, they became invisible.
The interesting thing is that as we speak, Chris and Richard are leading productive lives out West trying to explain the horrible person who let her dumb dog wander around at large and also seemed to think that Mom was some sort of slave put on the Earth to watch over her children because she was too fragile to deal. Like most people in this world, they don't have a lot of fond memories of the testy, fidgety, flighty and fragile lunatic with the screaming habit. There they were, stuck as victims of a dysfunctional marriage between a daydreaming mother and a loser father and they got no sympathy from the nasty woman next door when they acted up. They feel sorry for the girl who had to move in next door to be called a monster when she too was simply trying to cope with adult stupidity and also the youngest Patterson child who was clearly unwelcome in her own home. As for Elly, they're not going to lament her passing much.