I don't know how tall Arielle is, but a size 10/12 is pretty large for a 12 year old. And she has no idea what aging will have in store for her. Your metabolism slows as you age. If she keeps eating whatever she wants, in another 10 years, her weight gain will be rapid. I can honestly see her getting to Amberlynn levels of fatness easily. She needs to get it in control before that happens. She won't, but she needs to.
IIRC, Arielle is pretty tall, around six feet (which is actually an advantage, if you want to eat and stay at a healthy weight). If you look at her original tweet rather than the screenshot, and look at the full photo of her at age 12, she was indeed chunky for a girl that age, but not nearly as bad as now.
But Arielle has more reason than most to NEED to get her weight down to the normal range: she has repeatedly said she has arthritis, in which case it is vitally important that she reduce the stress on her joints by reducing the weight they must support. Which she refuses to do because she'd rather eat to excess, all day every day.
If I recall correctly -- this is true for all people, not only arthritis sufferers -- every pound of weight you gain results in a fourfold increase in pressure on your knee joints. Which means if you are 100 pounds overweight, you put
400 pounds of extra pressure on your knees, every step you take and every move you make.
Which is why young 20-somethings like Jude and Arielle complain they can't even climb a flight of stairs or stand on their own two feet for any length of time.
ETA: I just searched some earlier posts, and I discussed Arielle's arthritis (and the "one pound of weight = four pounds of knee pressure" statistic)
here. Including
this link to the Arthritis Foundation page discussing "How Fat Affects Arthritis," which includes this paragraph quoting a rheumatology specialist from the Mayo Clinic:
Every pound of excess weight exerts about 4 pounds of extra pressure on the knees. So a person who is 10 pounds overweight has 40 pounds of extra pressure on his knees; if a person is 100 pounds overweight, that is 400 pounds of extra pressure on his knees. “So if you think about all the steps you take in a day, you can see why it would lead to premature damage in weight-bearing joints,” says Dr. Matteson.