top dog

Question
I have a 14yr old gsd and a 2yr 2mnth old gsd both females, for 1yr 8mths they have got on fantastic my oldest dog was top dog and keeped the younger dog in line all changed last week when my youngest dog keeps fighting my oldest and now she know she wins she keeps doing this, we now have been keeping them seperated which is really hard but we don't want to stress the old dog any more as she is old and struggles to fight back (her back legs) I don't want to rehome my youngest as I love her to bits and I want to keep my oldest dog with us too as she has'nt got much time left now, any suggestions

Answer
It is only natural for a maturing younger dog to challenge a declining older one for status.  2 females are the combination most likely to be a problem.  I am not sure there is a lot you can do other than make it plain they are competing for second place because you are top dog.  See http://www.dogbreedinfo.com./topdogrules.htm