Ok well my sister lindsey?

i caught my little sister lindsey (6) looking at her cusin jacks (7) private part (he was showing it to her) what shuld i do??? im just 12 and my mom isint home and my aunts (not jacks mom) is across the street doing sumtin important shuld i tell her or shuld i yell at the 2 or punish them i need seriuos answers please