Sound's like you may be in a little bit of a tight spot ,but there is one thing you can do and I've used this tactic befopre myself and it does work .This is to find out who they are and explain to them that you have permiission as well to hunt this area and if you's could work together and hunt this particular area as a group or at least work out an agenda and let each other know when you's plan on being there, so as to not disrupt each other's hunt. This won't stop them from hunting the area ,but it will workable agenda to hunt this property together .
I don't see any other way around this and it may not be a total cure for your problem ,but it can work for both of you's .Since this may be a relative ,I don't think you have any other choice . If you raise a problem with the owner you may not have this spot to hunt any more ,but working together can help.

nubo