I agree with the above statements. In all my years of riding and working with horses, I've seen lots of close and of course not so closely related babies, and there is only one that actually had any serious problems. He was an Arab, his mom was bred back to her sire by mistake, and the baby was born with wobblers syndrome, and while he can't be ridden much, he lives out his life spoiled in a pasture, happy and healthy. Especially in Thoroughbreds and Quarter Horses, you can find a lot of "inbreeding" in the bloodlines of a lot of the horses, and they've all turned out just fine, and some have even gone on to breed and produce good babies themselves. I would definitely have her checked, just so you can have an idea of when she may be due, and so you can start making sure her nutritional needs are met while she's carrying, and nursing her baby. Good luck, and definitely keep us posted and put pics up.