Since you say her other dogs look fine, it's just her and her sister it's probably puppy demodex. Demodex is actually an immune system problem that is passed down by the mother.
One of mine had it and was on ivermectin for several months, she actually had a reacurence, but I think it was because we stopped the ivermectin too soon. She's fine now, but it took quite a while for her hair to grow back in.
Fortunatly in this day and age most mange is completly curable