I’m sorry but if he was my kid, I would let him go out and change the locks, the vile bully.
Yes he is still their family but so are the other siblings and grandchildren. Marcus clearly doesn’t want help anymore he wants enabling and maybe the kindest thing in the long term now would be to let him hit rock bottom , and see how he copes without mummy and daddy
overshare here but my eldest ( then aged 13) spend some time with a foster family last year because of his behaviour- basically a mix of puberty hormones, autism and anxiety that all exploded when he had to try and re adapt to going back to school, busier shops etc after lockdown ended .He was violent towards me and his sister, and smashed up our home . It broke my heart seeing him go but it was the best thing for him - he got the space to sort himself out with support , and he was able to come home a couple of months later .
If Marcus is allowed to keep acting like this without consequence , who is going to babysit him when his dad dies an early death due to the stress of it all.