。
She's sincere in both her actions and her words.
Literal
She [topic-は] action-also speech-also sincere is.
The pattern XもYも ('both X and Y') uses the additive particle も on both elements to convey inclusiveness — 'A as well as B.' Negative versions XもYもない mean 'neither A nor B.' 誠実 ('sincere, honest, faithful') is a na-adjective from Sino-Japanese 誠 ('sincerity') + 実 ('truth'); it carries a stronger ethical weight than 親切 ('kind') or 正直 ('honest'). The phrasing 行動も言葉も both conveys 'inside and out, end to end' integrity.