My mom, sister and I have been itching our heads for about 3-4 days but can't find lice.

It started with my sister about 4 days ago. We just brushed it off as her being strange. But then that night my mom started itching. And then yesterday morning I was itchy for a few hours and then it stopped, but now I’m itching again. Now we’re all itchy. We haven’t switched shampoos or anything like that lately, and it’s not like it’s allergy season. My mom has looked like crazy through our hair for lice but she can’t find anything. Any ideas? (oh btw I’m 13, my sis is 9, and my mom is totc)