What Brown Bear said.....FWIW I am a professional leather craftsman and have studied the subject for almost 50 years and the following is my advice based on those years of experience both building leather and repairing it.
I'm always hesitant to approach this subject since leather restoration of old and especially damaged leather is a complicated issue and when it comes to doing it right I defer to the experts - museum conservators. This is especially so when leather is an heirloom piece as this piece may be.
So in a case like yours, my first suggestion is to contact a professional leather conservator (I worked with one for a while and their methods can be very different than the common ones used by leather crafters -shoe repairmen and harness shops generally do not have the experience or methods to conserve leather - the methods used by conservators are based on tests done to the leather), but if that isn't possible or not your choice, than please read this very good article written by a conservator:
http://www.garciaaviation.com/conserve.html
With all due respect to the others who have answered, there is old dried out leather and then there is old DAMAGED leather and of the latter I've seen far too many pieces that have been ruined by folks (even experienced leather crafters who are generally not trained in conservation/repair techniques) using the wrong "tool" for the job.
For instance washing with saddle soap and/or adding a conditioner, which might work well on new leather or older dried out leather, can in fact cause damage i.e. old oil will acidify over time and adding more can exacerbate the problem and causes irreversible damage. Often times in old damged leather the cell walls have broken down and the integrity of the leather is lost - adding a conditioner in this case will often just increase the damage - I've seen it happen, what was once a prized possesion becomes a soggy, shapeless mess - no matter what the condition the leather is one should NEVER soak it in oil, that leads to destruction. In a case like that the piece needs to be stabilized rather than conditioned (or a combo of both with the conditoning if need be coming after the stabilization) and then stored properly (keep it out of the sun for one)
Personally - without examining the piece first hand I won't offer any suggestions other than those I've listed above...and FWIW even then I would still recommend a professional conservator.