Can my US born sister who has a learning disability but will turn 20 soon file for my foreign born mother at age 21?

Generally, a U.S. citizen who reaches age 21 then may sponsor a parent in the process of becoming a Lawful Permanent Resident. This is true regardless of whether the U.S. citizen child may have a learning disability. If the child does not have sufficient documentable income, however, then it may be necessary to have a joint sponsor in that process. Generally a joint sponsor may be any U.S. citizen or Permanent Resident who has sufficient documentable income and who is willing to assume the responsibilities associated with the Affidavit of Support. When the beneficiary (the mother) is outside of the U.S., then the immigration application process will involve consular processing rather than an application for adjustment of status. Of course, there are many other details that determine eligibility. There really is no substitute for your sister (with you, if that would be appropriate) and your mother to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options and strategies to attain your goals, and who then could offer legal representation in the often complex application process.