It is possible that your cousin's actions could be seen as violating the principle of delicadeza, which is a Latin American concept that involves showing respect, discretion, and good manners in one's interactions with others. Delicadeza involves avoiding actions that might be seen as rude, intrusive, or disrespectful, and borrowing someone's belongings without their permission could be seen as falling into this category.
However, it is important to note that the concept of delicadeza can vary from culture to culture, and what is considered to be delicadeza in one culture may not be seen as such in another. In addition, the concept of familism, which involves a strong focus on the family unit and a sense of loyalty and obligation within the family, may also come into play in this situation. Some people may view the actions of a cousin as being more acceptable within the context of a close-knit family, while others may see them as being more in line with the principle of delicadeza. Ultimately, it is up to each individual to determine what they consider to be acceptable behavior within the context of their own values and beliefs.