I am so sorry for your loss, honeybee.
You ask how to comfort your siblings- i am not sure of the answer, but please don't forget to comfort and take care of yourself too. Do you have someone who can sit down with your family alongside you to help tell them?
It sounds like a lot of the responsibility for your family/your dads estate/paper work has been left to you, do you have anyone who can help you with this?

Sending you huge hugs (as long as that's ok with you).