Definite articles in Spanish (‘el’, ‘la’, ‘los’, ‘las’) are often used in contexts where a possessive (my, her, their, etc.) would be used in English. This happens with body parts, or objects we can assume belong to the subject of the sentence. For example: ‘Sara se lava las manos’ = ‘Sara washes her hands’. We already know that Sara is mostly likely washing her own hands, so there’s no need for the possessive. However, the alternative ‘Sara se lava sus manos’ is, while a bit unnatural, not grammatically wrong whatsoever.