~~ Topic moved to Tourist Visa forum as this is about a tourist visa ~~
Because they didn’t abide by the terms and conditions of their visas. The visas were granted to them because they had to show that they intend to return to their home country after the studies / visiting as a tourist which they did not do. They believe your sister will do the same and she probably did not have strong ties to her home country.
No. Lawyers cannot do anything to guarantee an approval, especially for a non immigrant tourist visa