。
She is proud of her sons.
Literal
She [topic-は] her own sons [object-を] pride [as-に] is-thinking.
The plural marker ~たち pluralizes 息子 → 息子たち ('sons'). Japanese makes plurality optional; here ~たち is added because the sentence specifically wants to convey more than one son. The polite 思っています — distinct from plain 思っている — suggests a polite spoken or written register, perhaps speaking about herself to acquaintances or in a polite letter.