The bright side is she probably will gain some anyway given her family history so you don’t need to try much.
Her being opposed to weight gain is kind of pushing back against the inevitable. She’s prediabetic at a young age and it’s technically obese already, so she’s going to get diabetes at some point regardless of your influence, unless she turns into becoming a fitness enthusiast. She might as well just prepare for T2, honestly.
You could split the difference here, give her some healthy influences while still enjoying her being big without feeling like it’s something you caused, or even worse, having her resent you. The odds are overwhelmingly on the side of her slowly gaining anyway.
What does she say about her sister? If she sees her as kind of a cautionary tale, it’s possible that as long as she remains a little smaller than her, she’ll feel like she’s on the right track.