In the beginning when you first got her, the transition to 1 cup per day was a good ration to start with. But, now you can safely start upping her grain slowly. If it's been 2-3 weeks, then certainly she is ready for more, as she is nearing delivery and will need all the reserves she has. Start giving her a second cup at the "opposite" feeding -- if she's getting 1 cup in the morning, start giving her 1 cup at night. I don't know about Rosie's sister, but at this stage, she needs to build a reserve for the very hard work she is about to do. The additional cup is not too much for her to handle, as you've given her several weeks to adjust.
She's looking closer each day, and with a little rolling to move baby forward, and a little more elongation, she'll be ready. But please, give her a bit more feed since it's been 2-3 weeks which is plenty of time to make her adjustment.
Just as Anna's example -- and she has wonderful pastures so feeds less grain that I ever did -- since I had "dry-lots" here, my lactating and pregnant mares got 10-12 cups of grain, plus several flakes of alfalfa each day during their last trimester, and none were fat.