Yep, telling the parents would be a good idea. I would let your sister decide what she wants to do. Is she ready to have a child?
You did say she is off to become a flight attendant and so without a father, the circumstances are awkard - but there is, as others have said, adoption

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""The best love is the kind that awakens the soul and makes us reach out for more. It plants a fire in our hearts and brings peace to our minds."