Is it normal for my sister too eat this little?
Before I get told off: I'm usually not the one to interfere with my sisters life-style, but I've just spend almost a whole month with her and I'm worried. She seems to be paranoid to gain weight and I must admit she hasn't gained a single pound since she's been on her own. She used to be very obese, at 10 she was double the weight she should be and she turned it around herself starting when she was 12, now 8 years later she's still a bit chubby and it wouldn't hurt her to lose ~10kg (~22 pounds) but I'm worried since what she eats seems so little. She lives alone, if something goes wrong, there's no-one there to help her.
Per day she eats:
Breakfast: 1 coffee cup of Oathmeal, she cooks it in water, doesn't add anything but water.
Lunch: 2-3 pieces of fruit, apples, bananas, peaches, sometimes berries or pinapple. If she's having a busy or bad day it often ends up just being 1 apple. If she's really hungry she adds a handful of unsalted peanuts.
Dinner: a few vegetables (less than 200g/7.05 ounces) and on very rare occasions a bit of meat or 1 small potato when I practicly beg her to try it. When I didn't plan on cooking (ordered pizza or wasn't home) she never bought something for herself and judging from the dishes it's another coffeecup of Oathmeal or few more fruits, if anything at all.
She only drinks water and on Sundays she doesn't eat anything besides 1 coffeecup of Oathmeal before she takes her medication in the morning.
I've confronted her about it and she told me what she does eat is relatively healthy and it provides enough energy to get through her day, because she can't do anything. (Due to a disease that left her hips and back very weak she's unable to cycle/swim/do sports and she can walk very little, so her arguement of needing less energy than me is definitly valid, but I eat atleast 3 times as much on my days off) She won't really answer whether this is about losing weight or she just doesn't enjoy food anymore. She looks exhausted and unhappy, though I can think of another few reasons than food that can be the cause.
We both come from a family that consumes too much. Which according to her makes me unable to judge properly. Something I always took for granted since I can eat as much junk as I want and I won't gain any weight (I know it doesn't mean I'm healthy) but her boyfriend expressed his concerns aswell. So it's time for some more, unbiased, opinions.
Thank you for reading and hopefully your opinion.