Difficult one. Is her DH on board with trying to get her to see the issues? He might be the route in IYSWIM. Get him to talk to her first and express his concerns for HER. It shouldn't all be about baby as she might feel like she's being ignored in all this and he needs to make it clear that he's worried about her health as much as baby's. Him constantly 'nagging' however, won't help at all, as mentioned by others. He does have the right to tell her he's worried about her though, after all, he loves her. But one sit down and honest conversation might help.
If he wants to, he could make an appointment with his GP, or see if the MWs are willing to talk to him separatly so he can learn some tactics on how to handle this, (they may not even be aware there's an issue with food/body image), or they can at least reassure him that although LO is measuring small, there are systems in place to monitor everything etc. Ultimatly, at this point LO will take eveything it needs and it'll probably be your sister that 'suffers'.
I wouldn't be too worried about weightgain in itself, though this IS an indicator of issues, it doesn't always meas there IS an issue. I'm 25wks and have so far, not made it beyond my pre-pg weight. I just seriously don't want food, but am making myself eat healthily for LO. MW isn't worried so long as I'm eating healthily and no showing any ill effects (anemia etc). My only real issue is with tiredness, but it doesn't matter how much I eat...it's bad either way.
Anyway, as her waistline/body image is obviously an 'issue' and thus weight, maybe taking the 'eating healthily' route is a better route to go down than an 'eating more'/putting on weight route. It might encourage her to feel in control of the issue if you encourage her to plan meals for her and her partner? I know I found the whole you must eat x amounts of y thing utterly disheartening to begin with, but soon got over that after vommiting copiously from about 13wks to 21wks...holding anythign down felt like a small victory.
Trying to get her to graze might also make her take in more food than she's "realising" (even if it is "just" fruit/veg...try adding dips/homous etc to the apple or yogs to the fruit) try getting her DP graze throughout the evening, so it's not about her eating for baby, but THEM, but them doing 'normally'. Even if he initiates the eating whilst sat on the sofa together watching telly, you never know, she may join in?!
As for what you can do, other than being there for bro to vent on...well, I'm the kind of person that would respond with "no, you're pregnant, not fat and you DO look good" every time she came back with a negative. I personally don't like people "getting away" with making obviously negative statments about themselves...having been through pretty crippling depression, it's not always easy to spot how making little comments like that reinforce the negative feelings in my own noggin....so discourage others from it where possible!