That stinks. Sorry to hear that. I was in a similar boat with 10 mcs after my DD. Your sister should go to a reproductive endocrynologist (SP?) and get a full blood workup done. There are way too many to list, everything from genetic predispositions, to clotting disorders, to hormone levels and progesterone, to FSH which can tell you egg quality. Unfortunately, they only find a cause about 50% of the time if that. All my test came back normal, and after three long years I am finally 21w. Best of luck to your sis.
try checking her progesteron and estrogen levels