An only sister's wedding? I'd go no matter what the cost. If you said a cousin or a distant family member, I'd waiver . . . but you can't miss your sister's wedding. When you left the country, you knew that you'd occasionally need to spend on travel expenses to return home for family reasons -- this is an unmissable reason.
However, I do think that going without your family is an acceptable option.
And hopefully you can stay with family, so you're really only paying for the plane flight plus incidentals.