1. You petition mother. The process will take a couple years.
2. After arriving in the US, Mother petitions your sister. That will take several years.
@Boiler stated the only way for them to immigrate together if sister cannot be left behind. Sister's case, through only you, will take more than 20 years. I understand the problem, but, under current law, I don't see a faster way.
Your mother is considered to be an immediate relative of a US citizen. There is no waiting for a visa number, thus it is relatively fast ( a couple years).
Your sister is NOT considered to be an immediate relative of a US citizen. People in that category must wait until a visa number is available since there is a cap on the number of immigrants. There is a long, long line ahead of her waiting for a visa number. That's why sister case will take much longer.