Thank you for your response. Correct, she is not married. I was told by a lawyer that once my mother receives an approval letter for the I-130, she could petition for her.
What I'm understanding from you is first to petition for my mother GC first which might take 1-2 years (I dont know the timeframes)? then once she has a GC she can petition for my sister? By then she will be 19 or 20, so do you have an idea how long it takes to bring the daughter of a US citizen once the daughter has turn 21 and under 21?