Leather can become dry and brittle if not cared for properly. I recommend cleaning the braided items with a damp cloth using mild soap and water.
The horse gear may require a thorough lathering with saddle soap on occasion, followed by a conditioner (Picard's leather dressing or Vaquero rawhide cream are very good for tack).
After cleaning, I also recommend a good conditioner such as Leather Amore or Lexol on all other braided items. You can also use a light coating of oil such as neetsfoot or even olive oil (Not a joke). Keep in mind that the oil will darken the lighter shades of leather or may stain hats and clothing.
Stethescopes can be cleaned with anti-microbials and should be conditioned more frequently to prevent drying out.
For the wearable items, no harm will be done if they get wet occasionally, but care should be taken not to let the items soak for extended times (ie: bathing or swimming). With proper care, the items should last a very long time. I've been wearing one of the 8-strand 3-ply bracelets regularly now for over 5 years and it is still holding up well.