Your sister has no valid grounds to apply for asylum in the US. In fact, grounds for asylum are persecution by the govt for religious beliefs, political activities, etc. And the person must be fleeing "imminent danger" and apply at the nearest safe border. The nearest safe border to El Salvador is Mexico. She NEVER should have attempted to enter the US. You only have one shot at asylum, and once denied, have no recourse but to return home. She has to return to El Salvador.
You can go get her & take her back home to El Salvador with you. Or let her dig herself out of her own stupid mess.
BTW, dragging her kids all that way, through harsh dessert climates, etc, was extremely abusive of her children, and endangered their health & welfare. Instead of spending money on traveling, she should have spent that money at home for their benefit. What she did to her kids was downright criminal.