Also in my omaginery backstory M and G dated from teens and A was always the third peg with some girl or other so they're more like siblings than mumsnet bil/sil. A is older then M and feels bad thry went into that life together, he only did jail but M died so he feels extra responsibility. Maybe he even carried a torch for G back in the day.
T meanwhile always adored M and A but he was the baby bro, too little to join in. A went to jail and T followed in his footsteps to be like him and good enough. A comes out of jail a changed man and tries to change T who resents it because he's been gone so long, he's never good enough etc so all this with the moving away and now the house, it's T still proving himself to A bit he can't admit that even to himself.
And because G has been in Ts life forever she's def more like a sister than just the sil so he wants to look after her, step up for the brother he couldn't stop dying etc.
T is quite immature, I think they could have cast the actor younger but perhaps to get the right actor that wouldn't work, he doesn't look younger unlike Bella for example.
There no mention of their parents I'd there? A said about trying to get T on the right track etc but it's unclear if the parents are alive. I wodner if he could have been living with A and G in his teen years?