Is incest a joke

I don’t even know how to come out with this but my bf is 19 and he still lives with his mom, but I get very incest vibes from all of his family. His sister has asked him out as a joke before to prank their mom, one of the brothers slaps the sisters asses, etc and mom doesn’t do anything. She also still bathed the two youngest boys (13/11). My bf likes to pull down his pants where his full ass is out and his front pubes and sit of his mom and siblings. Today, he literally full of flashed his mom by laying on his back and putting his legs behind his head while his mom looked, she said it was gross but she laughs about it and they all think it’s a joke. Please tell me, is incest really a joke? And isn’t that so inappropriate for someone to do to their mom?