Uh, may I timidly suggest that you carefully examine the "need" to see the Aunt? If she is going to be awful to you, then why put yourself through it? That's a real question and there may well be compelling reasons, but I think it's worth having a conversation with yourself about it instead of just uncritically accepting the "need" as a given.
That said, if you decide to subject yourself to these awful people, my suggestions are as follows.
1. Any comments about your sister's weight loss are to be responded to with a smile and a "how nice for you." Repeat as necessary, a million times if needed.
2. Any comments about your weight or your appearance are to be responded to with one of the following (pick one before you go and practice):
a. A smile and a "How sweet of you to notice." Repeat as necessary, possibly with variants such as "Thank you for your concern" or "I'll give that the consideration it deserves." Inside your head, you are thinking, "These people are whack and this is THEIR issue, not mine."
or
b. A steely glare and a "I'm not comfortable discussing my weight and I'm not willing to continue this conversation. Where did you get that lovely brooch?" And then follow through by getting up and leaving the room if they persist.
or
c. A smile, a lighthearted laugh, and a "It's so amazing that you two are so worried about my weight and my appearance, because Biscuit and I happen to think I look just great! Don't we, Biscuit?" Followed by another lighthearted laugh. Again, repeat as necessary. I happen to really like this option. Don't try to convince them they're wrong and you're right, just state your opinion and make it clear that they're not going to change it.
Good luck! Big hugs!!