Is my brother a perv?

i have just seen my little brother look up our sister skirt. he was on the floor reading his mag and my sister stood in front of him. she had lost her ring and was looking for it on the sofa. as she looked, i noticed my brother have a peep up her skirt. my brother is 12 and my sister is 10. should i start to become worried about being in the same house as this guy? i would appreciate others with similar experience and is it just a faze my brother is going through?