winter
"I've told her that breast milk is healthier and gives the baby antibodies etc, but she won't listen to me. I gave her a baby book that explains why breast is best but she won't read that either."
That's where you are going wrong. By telling her that "breast is best" you are subliminally telling her that formula is ok, just not quite as good as breastmilk. This article explains it all much better.
To answer your AIBU question, no, YANBU, the truth is, if you choose not to breastfeed, for purely cosmetic reasons like worrying about it ruining your breasts (which it doesn't, by the way, the hormones produced during pregnancy are what change your breasts, not breastfeeding, so they are going to be different even if you never breastfeed at all) then yes, that is selfish. But don't be too harsh on her. She may have other, less fickle reasons that she is not able to articulate, or she may not truly realise the differences and risks involved in not breastfeeding.
I honestly, completely and utterly believe that no mother in the world would actively choose not to breastfeed if they really understood the differences and implications. Lots of women turn to formula when they struggle with breastfeeding, generally through lack of support or information, but I don't believe any of them think "I know this stuff is junk food for babies, and is going to leave my child with a reduced immune system and at higher risk of X, Y and Z illnesses, but I don't give a shit". No. No mother ever thinks that. They are misled to believe that formula is a genuine and equal alternative to breastfeeding, or are misled to believe that they have no choice but to use it (and use it feeling guilty for failing their baby, when actually they should feel angry at the people who let them down and didn't help them resolve their breastfeeding issues). But no-one, your sister included, would choose or plan to not breastfeed if they really understood the choice they were making. So whilst it is a selfish choice, it is almost certainly an uninformed choice, so the best thing you can do is give her information if she asks for it, and support her plan to breastfeed for the first few weeks and gently encourage her not to give up by pointing out how much more convenient it is etc.
I understand why it bothers you, but that doesn't mean you can bombard her with information no matter how well intentioned you are.