Genetic testing negative for downs...but one twin was found with a vsd- what are the chances of them having ds?

I am 26 and my husband is 29. We are 22 weeks pregnant with twin girls.  All of our initial genetic testing has come back negative for any abnormalities. However, last week it was discovered that one f our twins has a vsd in her heart - which is known as a soft marker for Down syndrome. The doctors have told us the only way to know for sure is to do an amino. My husband and I are completely torn up about what to do and have been trying to research the chances of them having downs when the only marker is the vsd. I am an elementary teacher of students with intellectual disabilities so I am not looking for a lecture on how to love a child with disabilities. Simply wondering if anyone else has dealt with this and what the chances of them having downs could be when the testing came back negative from first and second trimester screening.