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It is certainly possible that your sister could have twins, but the odds are against her. Despite all the notoriety, twins (and higher order multiple gestations) are quite rare. Dizygotic (fraternal, "two-egg") twins do run in
familiesBirth control and family planning
Choosing a primary care provider
Ewing’s sarcoma
Family troubles - resources.
Ultrasonographic evaluation will definitively tell you if twins are present. Other things such as a uterus large for dates, etc., can provide clues but all things considered, ultrasound is "where the money is".