I assume you mean form 1098-T, from a college. The 1098-T is only an informational document. The numbers on it are not required to be entered onto your tax return. So, first be sure Sis actually claimed some benefit on her return. Look at lines 8a & 13b & 18c on her form 1040. If they are blank or 0, she didn't claim anything and doesn't need to amend her return.
Mom can go ahead and claim her educational expenses. This assume Sis qualifies as a dependent on Mom's return. Even if Sis does need to amend, Mom can go ahead and file, claiming the education credit. She does not need to wait for Sis to amend. If Mom's e-file is rejected, she will need to mail in her return.
There's a new urban myth among college students that says they can get a $1000 from the government just for filing a tax form. For most of them, they simply aren't eligible. A full time unmarried student, under age 24, is only eligible for the refundable portion of the American Opportunity Credit if he supports himself by working. You cannot be supporting yourself on parental support, 529 plans or student loans & grants. You must have actually paid tuition, not had it paid by scholarships & grants. It is usually best if the parent claims that credit.
You cannot claim a credit if you are, or can be, claimed as a dependent by someone else.