Help me Understand 😞

Soo I have a 5month old , he’s my 🌈 baby. I had a miscarriage @17weeks before him, The doctors advised I get put a cerclage for my son, I did and all went well… now my younger sisters both got pregnant at almost the same time being two weeks apart.. they both just had premature labor one @26 three weeks ago, the other @27weeks a week ago, Their babies both lived for just a few days, didn’t make it. The doctors don’t have any explanation for what the problem could be , all they advised is to both use cerclage in their next pregnancy. My question is : is incompetence cervix genetic??