So sorry to hear that your sister miscarried. And also that your dream of being an aunt was lost.

From what I read, miscarriage is an unfortunate risk associated with PCOS. Taking the metformin was the best precautionary thing that her doctor prescribed, but there is data showing some possible safety issues. Most women with PCOS are most at risk of miscarriage during the first trimester - but there are so many other factors that could cause it to happen later.

Hopefully the doctor will be able to determine the cause when the D&C is done. Not that it brings back the baby, but perhaps there was some other reason - all beyond anyone's control.

Please voice your support to your sister. Don't ignore that the miscarriage happened. Let her know you love and care about her and grieve her loss as well.