I agree completely.
The condition she is currently in leads me to believe she will not be an "easy keeper". Old horses teeth pass a certain point where they no longer can eat or process hay/grass for nutrition. They HAVE to be supplemented with senior feeds, and often with chopped forage or soaked cubes in order to get "long stem roughage" to keep their gut working right. She is a mini, so the amounts she will need to eat will be much less than a full size horse, but she WILL NOT fit into the feed budgets we have talked about in other threads. You will also be looking at a lot more time invested in separating her during feeding so that Xenia doesn't eat her food. And also soaking the food takes time, especially cubes!
If her foal is a yearling, there is NO REASON for her to be thin like this. A yearling foal should be eating plenty on his own, and not draining his Momma. Also, the fact that she hasn't shed out slick by this time of year is a good indication of poor health or the beginning of metabolic issues.
I think she would be a fantastic companion for Xenia, and a good older experienced horse is invaluable for showing a green owner "the ropes" in a calm and patient manner. You just *REALLY* need to think about how she will impact the budget presented to your parents.