Aunt Juley is the kind of comfortable, uncomplicated character we expect from Forsterian aunts. Aunts play an odd role in general in his novels – they seem to be ever present (often more present than mothers or fathers), and they're either consistently good or consistently annoying. Luckily for the Schlegels, Aunt Juley is the former, rather than the latter. She's a sweet lady, who seems primarily occupied with keeping her nieces and nephew happy, and (in her mind's eye) providing them with sound advice in the absence of their dead parents.
Aunt Juley doesn't have her own background or her own story; rather, she's just here as support for the Wilcox-Schlegel drama we see unfold in the novel. While she's inadvertently the catalyst for the whole business in the first place – after all, her altercation with Charles at Howards End basically sparks this whole sequence of events – her role after this initial moment is inessential. Really, she's most useful as a kind of cheerleader, both for the Schlegels, and for England. She represents the most predictable kind of old fashioned English national character, the same way that Frieda represents Germany. The England we see in Aunt Juley is comforting, kind, a bit outdated, and, for lack of a better word, auntish.