Sadie
I just couldn't make out why Sadie hated Selina so much? In the very first scene she shows up, she expressed a great disdain towards Selina. Did I miss something?
Is this better explained in the book maybe?
I think she's just one those people who should never reproduce. I doubt it was anything that Selina did. Her mother was too self absorbed to care for anyone, but herself so she resented having to be burdened with a child.
Mecca
In the book, Sadie is portrayed as a nasty, bitter washed-up prostitute. She schemes with Rose-Ann to join her in starting their own brothel, with Selina as both a maidservant & as a "curiosity" for their johns.
Sadie ridicules Selina for being blind--because Selina can't see her, Sadie pretends she can't see Selina (i.e., "Dig that crazy ghost!"). At one point in the story, she actually trips Selina knocking her to the ground--which makes both Rose-Ann and her trashy pal howl with drunken laughter.
Sadie's disdain and treatment of Ole Pa is equally as viscious. Their exchanges are venomous:
OLE PA: "You here again, ya big fat slob?"
SADIE: "Yeah, and from one *fat slob* to another...how come youse still ain't in the nut house? How come a diseased old pooodle like you hasn't been picked up by the dog catcher, huh?"
A truly vile character [but memorably portrayed by Elisabeth Fraser]