We have to remember animals don't show pain like us humans, and I'd say she is in pain. Not putting weight on her leg for one. and not pushing back. When you lay her on her back for just a sec. and pull gently both legs down straight do you notice anything different with the injured one? Best i know to do is let her rest and move around in the brooder only. Try not to make her get up an move let her decide what she wants to do but do give her warm water therapy at least a couple times a day. Even if she only floats the warm water will keep her muscles relaxed. Like I said leg injuries sprains etc take a while to heal maybe a week or longer. But if you notice any change in appetite or her showing no interest in her sis then time to get to a vet. or even now if parents are in agreement. I'd hate to say yes let her get over it and something be seriously wrong. you all are there and see what she is doing. Duckling have soft bones when young so hopefully this is just a sprain and she'll bounce back quick.
