Its difficult to say whether your mother and 7 year old sister will get visit visa, it depends upon the consulate officer and their personal ties with the home country.
There are multiple factors that determine whether a visa application would be approved. If you want to know how your mother should try to convince the consulate into issuing this visa you should contact an experienced attorney. Best Wishes!
Lalita Haran
Attorney: Immigration Law
13295 Illinoois St. Ste 128
Carmel IN 46032
Ph: (317) 660-6174
www.haranlaw.com