walked in on brother

so i just walked in on my 13 year old brother masturbating to anime on porn hub and i’m literally going to vomit i cant get the image out of my head... i didn’t know he did this and it’s not that i don’t think it’s normal i just cant imagine my baby brother doing that stuff you know? anyway what do i do? i really feel like i need to talk to someone mainly my mom and i know she won’t tell him i told her, but i still feel bad and i don’t want to embarrass him. what do i do? also how tf do i act normal tomorrow? i see him all day at home and agh this is just so nasty.