chocolatehobnobs, your sister does sound in general like a bit of a pain. It sounds as though you have issues going back years and a whole host of things she's done, and will no doubt continue to do, that wind you up.
But consider for a moment that of all these things she's done over the years, the one you've singled out to start a thread about is that she doesn't cover her breasts with a scarf when she breastfeeds. Even everything else that happened in Wagamama didn't make it into your OP. This does suggest that it isn't all about your sister.
Going back to your original question "AIBU to want to do things differently or is she right?" the answer is both. She is right that carrying on when breastfeeding exactly as you would if not breastfeeding is being a good role model. If breastfeeding were normalised like that it would improve initiation rates, improve likely success rates, and IMO cut back on the pervasive pornification of women. But at the same time YANBU to want to do things differently. You should feed your baby in the way that you feel comfortable, just as she should, and if you want to cover anything at all with a scarf then you should go right ahead and do it.
HOWEVER (I am not shouting, just adding emphasis to that word. It's only when you put whole chunks of text in capitals that you are shouting) either she's not behaving perfectly normally when breastfeeding or her idea of "normal" (butting into strangers' conversation to lecture them on something you know nothing about yourself) is something that would make other people uncomfortable whatever the state of her breasts when she did it. In fact, if you'd started a thread on "Is my sister BU to interrupt complete strangers in Wagamama and tell them what to order when she's only been there once herself?" you would probably have got near-complete unanimity that she was BU. And being strange and pushy while breastfeeding isn't really being a good role model -- but because you are being strange and pushy, not because you're being strange and pushy while not wearing a scarf .
Think about the "choose to try to consider other people when out" point. Some people don't like to have anyone black or Asian sitting near them; should they be "considered" by avoiding eating in the same establishment if you are black or Asian, or perhaps by draping a blanket over your head so that your ethnicity isn't obvious? Some people (a disturbing number, actually) don't like to have anyone obviously disabled in a non-picturesque fashion sitting near them when they are eating; should they be "considered" by the people with disabilities or (if appropriate) their carers? At the other end of the spectrum, most people would probably dislike having a cockroach-racing contest staged in the coffee shop at lunchtime, and that probably should be considered. It's not that you believe in trying to consider other people when out and that everyone who disagrees with you doesn't. It's that those who disagree with you see breastfeeding as closer to the black/Asian/disabled end of the spectrum where it is unreasonable and fundamentally unnatural to object to it and the offended person really needs to work on an attitude adjustment, while you presumably see it as something in the middle where being offended or uncomfortable is a perfectly valid and value-neutral position. I am considerate of other people in all manner of ways when out with my children; that just doesn't extend to covering all visible bits of me below the collarbone up with a scarf when breastfeeding the youngest.
There is a marvellous thread on MN somewhere from someone who had had a waiter come over to explain that someone had complained that they were offended by her breastfeeding her baby. But she wasn't breastfeeding. In fact, she had formula fed from birth and had never breastfed. She was just holding her sleeping baby. The complainer was just so weirded out by the whole idea of breastfeeding that he/she managed to create an entire public breastfeeding scenario in the depths of his/her imagination and then offend him/herself with it.