My sister has bleeding during pregnancy. What should I tell her?

Bleeding. There are many causes why one bleeds in pregnancy, some of them could be life threatening. The only safer advice is to have her see a physician immediately.
Pregnancy bleeding. It's difficult to answer this question with such limited information. Is this an early pregnancy (1st trimester)? In this case some of the causes could be miscarriage, ectopic pregnancy, molar pregnancy, or infection. If it is later pregnancy placenta previa, placental abruption, preterm labor, etc. Are possibilities. Tell your sister to see her doctor so a complete workup can be done.