AITA for telling my mum to "cut of the umbilical cord already?!"

Hello reddit so this is my first time doing this and im not too sure how this works but heres nothing

For context I F 16 am the eldest out of 6 siblings, F 14, M 13, M 8, M 6 and M 5. Now M 8 was my mothers rainbow baby and she's always coddled him when he was younger what was fine but it just got worse as the years went on and he's now starting to show signs of being obsessed with her whenever any of us talk to her for more than 10 minutes he will glare at us and start to do anything to have the attention on him again. It's ok if he leaves the house to e.g. go shopping with my dad school or going round a freinds house but a soon my mum steps one foot out of the house he has to know where she's going who she's going with and how long she'll gone and if she's gone for more than 12 minutes he'll start to kick and scream until she comes home and she'll coddle him and get him whatever he wants. It's got to the point where none of us kids want to be around our mum because he has to be there otherwise he'll have a meltdown even my dad has had enough but he doesn't want to start an argument with my mum But this past week its been unbearable and it got to the point where I yelled at my mum cut of the umbilical cord already?! So am I the asshole??