Hi , I just got my citizenship and I was wondering how do I go about, if I want to bring my mother (age-51) and my sister(age-19)They live in Eastern Europe. I am pretty sure I can fill out some forms for my mom, but I am not sure for my sister. Thank you very much for any of your thoughts.

Yes, you can file petitions for both of them; sadly your sister's won't be "current" or available for use for about 12-14 years whereas your mother's will be "immediate" (processing time mostly - say 3-6 weeks, depending on embassy workload).
Siblings of Amcits have a long long long wait before they can get an immigrant visa - this is due to a short supply of visas in this category and oversubscription (or "overbooking", if you will).

You should file right away for your mom and file a F4 peition for your sister. Let us hope that your mom gets here before your sister's 21st birthday and she can file a F2A petition for her before she turns 21. Either way, it will take a long time but she will get here.

Good Luck