。
I wonder if she'll marry him.
Literal
She [topic-は] he [with-と] marry [explanatory-の] [I wonder-かなあ].
~かなあ (often spelled ~かな) is a sentence-final 'I wonder' particle — soft, casual, and often spoken to oneself. The の before かなあ gives an explanatory feel: 'I wonder if (it's the case that) she'll marry him?' The lengthened あ adds reflective drawn-out musing.