Sarah is so funny to me; Silas doesn't get along with Marcia because they have an EOW Apprentice Rivalry and they must bicker about it or they'll die. Sarah, seemingly, just doesn't like Marcia because she is taking up Septimus' time (and Marcia is arguably as much as or more his mother than Sarah!) and she's jealous. I like to think that, ironically, the more positive influence Marcia has in her kids' life the more she can't stand her.
Sarah learns from Jenna years later that the reason Boy 412 (Septimus!) had come with them to Aunt Zelda's in the first place was because Marcia dug him out of the snow and saved his life and carried him all the way upstairs in her arms. Then she remembers that Marcia had also saved Jenna from the assassin, twice, and something wars inside of her between immense gratitude and so much annoyance that it was Marcia of all people and she just sits in silence for minutes while her face grows red.