Letting people know about how much the pet industry screws with these animals is good an all, but it dosnt really answer the question...
Get it to a vet, would be my answer, though as I know, a good vet can charge $100 to take a look at an animal... im guessing you paid FAR less than that for the animal to begin with, so Im gonna guess thats just not an option...
...if im wrong, TAKE IT TO A (GOOD, REPTILE SAVY) VET.
Otherwise, you're options are REALLY limited.
First off, try to find someone close by that can take a look at the animal and maybe spot something you havent.
Try offering different types of food, as maybe the animal isnt acclimating properly and you're offering an unrecognized food source.
Check temps again... make sure they are right on.... hell, check it twice.
What kind of substrate are you offering? What kind of hides? What kind of caging?
You really need to offer more information.
If the belly is distended, it may have some kind of blockage. Make sure it has plenty of clean water to drink, as from what ive been told blockages/impactions usually result from dehydration.
And lastly: LEAVE HIM THE HELL ALONE!
Dont handle him, stay as far away from him as possible for the majority of the time- hes stressing, hardcore.
He needs time to acclimate himself and get used to this new environment. His health is likely already on the down-hill side of the grade, so you need to give him time to himself to figure things out. Offer food, offer water, provide proper husbandry and tell your sis to back off and let him do his thing in private.... aside from taking him to a vet, thats about all I can suggest.