My sister lost a lot of weight and I react poorly over it.
Why did you react poorly?
She was over 220 Kilos, now she's down to 210 kg, it sounds exaggerated.
That's a significant weight loss, but it sounds reasonable. You should offer encouragement and support to sister instead of being skeptical about what she has achieved. Encouragement will motivate her to keep trying to reach her goal.
It isn't possible to have so much weight loss. She has just lying about it.
You should support her, whether she is lying or not. Clearly something is going on in her life where she either wanted to lose a lot of weight or make people believe that she did, and if you love her then you'll let her have her moment.