Babies notoriously arrive on dates other than due dates..those dates are, at best, guesstimates. As your sister has said that she doesn't need you right in the room with her, but afterwards, I would think that if you got there even 5 days after the baby was born, it would be fine. You have the rest of your life to be there with them. Your dh and kids should be first and foremost for you. You have made plans, important plans. The rest of your family is counting on you to go with them. To ask everyone to change their plans in order for you to be with your sister after her baby is born is a bit much. I understand wanting to be there, for her. But in all reality? Being with your dh and ds are more important. Surely your sister has her dh to support her for the few days you might not be there.