Certainly sounds like your sister has a metabolism problem, so your plans for tomorrow are in order. I suggest that you insist that the doctor run a full panel of thyroid tests, with emphasis on free T3 and free T4, along with TSH. The reason for this is that free T3 and free T4 are the active components that largely regulate metabolism. In addition, free T3 is 4 times as potent as free T4, so it is the more important test. Doctors will usually just run TSH and total T4, and sometimes total T3. This is not enough to really evaluate thyroid function. So insist on the frees.
When the tests have been completed, make sure you get a copy of the actual numbers and reference ranges. Don't just accept from the doctor that everything is "normal". Then, post the numbers and ranges, along with the doctor's assessment, and let the forum members give you their assessment of the results.