、。
'I hope mother isn't angry,' she thought.
Literal
Mother [subject-が] not-become-angry [if-ば] good [but-が], [quotative-と] she [topic-は] thought.
The quoted thought 怒らなければいいが is a hope-expression structure: [verb]-なければいい = 'it would be good if X doesn't happen' = 'I hope X doesn't happen.' The trailing が is a sentence-final softener conveying hesitation or worry — '...but it might happen anyway.' Not to be confused with the subject marker が. The whole construction, with its ば + いい + が, is a compact way of expressing anxious hope.