If Kim and Sammie are related and both of them have children who were profoundly disabled at birth, plus a slew of miscarriages between them, seems it's easy to draw the conclusion that they come from a line with serious genetic fuckery. Most miscarriages are due to some sort of anomaly in the fetus and not anything the mother did wrong. So if Kim is having multiple miscarriages after a live birth with disabled child, it's weak genetics. Also if Sammy has an incompetent cervix, she probably has had multiple pregnancies that end in miscarriage too. Incompetent cervix doesn't prevent pregnancy, but without surgical interference, the pregnancies don't stick. If that's not the case, it's probably also due to having genetic abnormalities.