Do you think that I should "out" my sister as straight?
What do you mean by outting your sister as straight?
I mean she's straight, my parents do not agree with her sexual orientation
If your sister has a sexual orientation that she wishes to keep hidden from your parents, you should respect her privacy and not reveal that secret to your parents, especially if they disapprove. Sexual orientation isn't something that people can change and it's wrong of your parents to not agree with whatever sexual orientation your sister follows. Instead of stirring up family drama that can hurt your sister, keep her orientation secret and try to support her as best that you can.
My sister can't live her life that way, she needs to realize that it's the 21st century and nothing for the parents to blame her for.
It doesn't matter what reason you have for outting your sister, that is her place to reveal that and she should choose when. How would you like it if she revealed some huge thing about you?