I would imagine JC and the Youth would only prevent her from leaving *after* an incident where Logan was severely harmed, not before. To my mind, that was the vital pivotal moment where when the Youth stopped her and she returned, they all knew they were all 'in it together' covering up horror from there on in.
I'd be interested to know what this 'leaving' comprised and how determined it was, if at all. Was she carrying her baby? Her handbag?
She could have got away, she could have screamed out to the neighbours, the man in the van, she could have fought the Youth off tooth and nail. She's a very loud mouthy person by all accounts. But instead, she made a choice, a decision right there - a decision to stick with her family at all costs, at Logan's cost. Logan might still have been alive but he was confirmed dead right at that moment.
From seeing it this way, I believe that is the 'X' moment that marks the collaboration and means all three jointly murdered Logan. JMOO
Edit: if she'd tried to leave before the severe assault, they probably would have laughed at her and said 'see you when you get hungry' or some such