Here's the sad truth - we can't change parents' behavior. To be perfectly honest, the only behavior we can control is our own.
But the good news is that you can be consistent and it WILL help. By 3 yrs a child is old enough to understand that different places have different rules. I can run in the park, I can't run in church. Not that they'll always follow the rules, but they can differentiate. Keep being consistent with both children and they'll come around. For you. The parents are probably SOL.
