Kids at that age tend to live in the 'now', not the past or future. Plus, they're still talking to their mum, even if they're not seeing her. Not every kid is like this, of course, but be thankful for small mercies. If the boys had significant psychological issues as a result of the accident, things would be that much tougher for Alan and Peter. It's up for debate, but I think Lisa made the right call in forbidding her boys to come and see her. If they saw her regularly while she's still badly hurt, it would definitely affect them.
As for Peter, sadly this is not an uncommon story. It's the reason relationships sometimes break up after a child is born -- the new member in the relationship disturbs the balance, and sometimes they disturb it enough to end that relationship I wish that wasn't true, but...