回复：请问我大姨61岁了， 想来美探亲，

If your grandmother or even your mother had filed for her GC appication in 1991, her GC priority date is up already and all her family (including minor children) can now in US holding GC.
As for B visa, really what you have already known, she needs to show that she will go back to China after this visit, bring whatever can support her claim of no immigrant intent.