If two suitors expressed interest in my sister at the same time, and both appear equally suitable according to her wali, can the sister do the نظارة شرعية with both before rejecting one so that she can gauge who she’s more compatible with after meeting them?

‎Yes, as long as neither has been accepted yet, she may consider both and do the viewing with each, with a mahram present. Two men proposed to Fatimah bint Qays raḍiyallāhu ʿanhā and she consulted the Prophet ﷺ‎ before answering either (Muslim). Once she agrees to one, the other may no longer proceed.