My mother was delighted by the news, and so was I.
Literal
Mother [topic-は] that news [object-を] was-delighted-[polite-し], I [topic-も] so was-[polite-でした]
喜んでいましたし = 'was delighted, and...' — し links related clauses. 私もそうでした = 'I was also so' — extends the same state to the speaker. Polite register throughout.