Broodmare duties changes a body permanently. A rider has to become accepting of this fact. For a broodmare to return to active dressage duty is going to be a challenge. Not only does the mare need time to rebuild her topline, but her abdominal strength as well. The abdomen supports the topline! Because the muscles and ligaments have been thoroughly stretched from pregnancies, they don’t really bounce back. Perhaps after 1 or 2 foals, but each progressive pregnancy reduces the “bouncing back” abilities. It could take a couple years, but she might never return to a svelte, lean, mean, dressagy machine, uh, mare. 
If your sister is willing to accept the mare’s new body and just be happy to get her as strong and physically fit as she can be, then that’s fine. Just don’t expect her to be able to do First Level right away, let alone Second and least of all Third. It might take her a good 3 years to be physically fit enough for 3rd level. On the other hand, she might surprise you all and zippity-doo-dah she’s right as rain for Third in relatively short order of a year or so.