What are the boundaries of the relationship between a girl and her sister's sons (her nephews), in terms of clothing, gatherings, and conversations, taking into account the strong relationship and mutual familiarity?

The sons of a sister are considered mahrams (unmarriageable kin) to their maternal aunt. Thus, it is permissible for them to be in seclusion with her and to speak with her, as long as there is no fear of temptation or lust. However, if there is a fear of fitna (temptation), then it becomes obligatory for her to veil herself from them.