Unfortunately that's not possible. I am telling you what a lawyer would tell you. The best course of action so your sister won't suffer alone in her country is to file petition for your father and leave your mother with your sister.
When your father becomes a Green Card holder, then he can file for them both and it would take about 7 years for them to get their visa, BUT in 4 years and 9 months he can become a US citizen (5 years of taxes required, 1 can be excused), then all he would have to do is to get an update for your mom and sister to USCIS. They would be here 6 months after that.
Just an idea.