Microwaves can generally reach up to nearly 3/4" in water. If heating 8oz in a glass, that should still heat the water evenly enough, since the microwaves will still mostly pass through the glass.

Heating in shorter increments and stirring in between should help avoid breaking the glass, and heat the water more evenly. If you take the time, it should be identical to kettle boiled water. That's why people prefer a kettle to a microwave, it's much faster and far less effort.

But the sister also performed that experiment in the worst possible way, why did she microwave the actual tea bag?!