You can start with your sister or your mother or both at the same time. There is no order you need to follow. Having said that, often the sibling takes so long that sometimes it is quicker to bring the parent here and wait for them to obtain citizenship and have them apply for your siblings. Having said that, I would advise that you apply for your sister and mother simultaneously.
There is no order for petitioning relatives. You must file I-130 petitions for each your mother and sister. Your mother will be able to come first. Depending on whether she is eligible for adjustment of status or not the application process takes from 3 to 12 months for her. Your sister will have to wait approximately 10 years to be granted resident status through you.
I agree with Mr. Schmidt's answer, but would add that petitioning for your siblings would realistically take as long as 13-15 years. Depending on the country of origin, the family members should also apply in the DV green card lottery every October.
You can start with either one or both. If you apply for your mother, she falls into the immediate relative category and the visa is immediately available although processing is much longer depending upon where she resides. Your sister is a much longer process due to quota restrictions, but you may start the process. My colleagues point out correctly it might take 10-15 years, but in some circumstance your mother can petition for your sister too once she gains permanent residency and this second option might end up being faster. I suppose the point is you might consider several routes at the same time.