So for the past few months her boyfriend has been ringing me up and shouting random shit down the phone. A few examples are
me - "hello?"
him - "CHASING PIGEONS - I SAY CHASING PIGEONS"
and the phone goes down.
me - "hello?"
him - "Lets all buy a house"
me - "sorry??"
him - "its my fucking house and you'll never take it away from me"
phone goes down with lots of laughter in the background.
me - "hello?"
him - "I just had a massive poo and now my bum is hurting"
phone goes down and I can hear sister howling with laughter in the background.
These examples are just the ones I've had this week! like I say it's been going on months. Sister thinks he's hilarious. My mum says not to say anything to sister as she'll "get upset" 
AIBU to think he's obviously a bit of a twat and should be told?