My older sister has t2dm, is taking oral hypoglycemics, but is still finding her bgls are rising as high as 15 fasting. Should she be taking insulin?

Quite possibly. 15 (270 mg/dl) is quite high and she needs something changed, whethe it be higher doses of her diabetes pills, adding more pills, calibrating her blood sugar meter, adding insulin, she needs the sugars to improve. She should also meetwith a dietician to improve her diet, bc drastic bg changes can come from cutting down on foods that sneak sugar into your day (syrups, candy, bread, soda, ketchup).
Maybe. You mean her fasting blood sugar rise to 150? Or 15? I assume you meant 150. The answer depends on what her bs do the other times (before, after meals...) what is her overall control? If her numers are usually < 180 and her A1c is around 6.5-7.0%, then the oral hypoglycemic is working and she doesn't need to go on insulin.