The second season episode "The-Not-So-Ugly Duckling" (#34) marked the incipience of a trend that only heightened in intensity as the show went on: Marcia got her bitch on with Jan.
While combing her finely conditioned golden tresses, Marcia calmly informed Jan that, "If boys don't find you attractive, don't blame it on me."
This happens, of course, after Marcia has callously interrupted a study session between Jan and Clark Tyson, the boy who drives Jan to freckle removal, wigs and other such vices of unrequited love.
Just look at the way Marcia slinks into the room, in her polyster "fuck me" outfit and assorted Jezebelian accoutrements. She knew Clark was hot for her, but he was as disposable as Alan Anthony and Harvey Klinger. Marcia intentionally caught Clark's eye solely to rub lemon juice in Jan's freckles.
In "Her Sister's Shadow" (#59), as Jan is giving the famous "Battle of Marcia" monologue, Marcia conveniently storms into the den. She must announce that she's been made editor of her school newspaper just as Jan is whining about how Marcia has everything handed to her on a silver platter...a silver platter for which engraving is quite expensive.
Though Marcia had her occasional rise-and-fall (the Juliet debaucle, that tacky lie to Charlie about something suddenly coming up, the failed driver's test), it was Jan who bravely persevered and failed with Tiger, boys, bicycles, basic math, cheerleading, and even being Miss Popularity for God's sake. And with each failure, Marcia was there to say, "Oh Jan, I did it so much better" (usually conveyed with a mere glance while combing hair or reading Teen Time).
Of all six siblings, Marcia and Jan's dynamic was the most complicated. Theirs was a complex, finely constructed dichotomy of trust and friendship mixed with layers of contempt and jealousy. Behind both white smiles lurked the markings of true, unbridled teenage angst and agony.