I don't want to throw cold water at you, BUT, if they seem like they are getting brittle, they may well be.

I work in a museum and I can tell you that bone will get dry and brittle and there is little that you can do to prevent it. Linseed oil is a very bad idea. This will make them look moist, but will in time further weaken the material. If the bone is looking dry and brittle, set them aside and get a new pair of real bone "bones".

In talking this over with our research specialist (who used to dig nd cataloge bones) she said that they will dry out and loose much of their ability to absorb shock. Glues will help for a while, but later they can cause other problems that may make the bones no longer fit for use or even for display.

You best bet is to put them in a safe place where they can be admired, and then play the new copies.