I am so sorry that you have received this terrible news smudge.
It might help to try putting yourself in her shoes and thinking about how you would feel if you were in her position.
I think if it were me who had received that devestating news and was about to receive visits (possibly last visits) from family and friends, I would like them to be as honest and as 'normal' as possible. What I mean is that if I had a friend/relation who was an emotional sort normally then I would want/expect them to be tearful. But if I had a very straight, matter-of-fact relation, I would want them to be able to ask the straight forward questions that maybe others wouldn't ask. Also, some family members are natural clowns and it would be weird for them to come and visit me and be able to naturally deal with the whole sitaution without a joke or a laugh (if maybe a nervous/sad one).
What I am trying to say is that I reckon your aunt would want you to be just who you are - and the same with your sis.
If you need to cry, then cry - she will understand. You are sad and she doesn't expect you to try and pretend that you are not. I have been with a lot of people who are terminally ill and my experience is that they learn 2 things at the same time ie how to deal with it themselves, and then how to help other people through it. As much as it has been a horrible shock for them, they are aware that they will soon be falling asleep (often relieving horrible pain) but that they are leaving behind many many people who's hearts have been broken.
I wish you, your aunt and all your family the very best.
Take care.