I would have the mare checked for Lateral Patella Luxation, it is a ressesive genetic disorder that is common in Miniature horses and shetlands both in the US and the UK.
Inbreeding is not something I would do myself ever and now I am dealing with problems caused my inbreeding in a way, my stallion has Lateral Patella Luxation and compared to a lot of horse and ponies he isn't that in bred, however it has caused a genetic fault that has been hidden in his lines to come to light. He is bred from the best lines in the breed and some lines are very common making them impossible to avoid. This is the reason I wouldn't inbreed as there is this genetic problem hidden in my breed and there is no genetic test for carries so we have no idea what lines it is in so to avoid it you avoid inbreeding.
I bet none of that made senses anyway.
Here is my stallion pedigree
Enrique of St Ninian Shetland Pony
Our foal by him also has it and the parents weren't that closely related.