Assuming you are a US citizen who is 21 years old or older, yes, you are entitled to petition for your mother and 12-year old sister. The fundamental problem, however, is the timing of an available immigrant visa slot for your sister under the current Regulations. While your mother is considered an "Immediate Relative" under the existing Regulations and, therefore, not subject to the immigrant visa preference/quota system and could be admitted relatively quickly, your sister falls into classification F-4 (Brothers and Sisters of Adult US Citizens) for family-based sponsorship. Depending upon what her nationality is, she would currently have at least an eleven-year wait for immigrant visa availability as per the June 2012 USDOS Visa Bulletin once a priority date has been established for her through the filing of an I-130 Petition for Alien Relative (assuming there are no substantive changes authorized by the US Congress in the immediate future regarding an expansion of the number of available family-sponsored preference visa slots in the various categories).
Answered on Jun 07th, 2012 at 8:35 PM