I'm planning to rat out my sister to our parents about her relationship with a much older gentleman
oh dang! Do you think they will have a problem with it?
I'm sure they will be disappointed about it
It's wrong that your sister is hiding the truth about her relationship from your parents, and depending on how old she is, this might not be allowable at all. However, you should not rat her out, because this will just cause problems between you. Try to convince your sister to be honest or, if necessary, to break off the relationship.
Nah, I don't care how she will be effected. It's my parents who need to know the truth.
I think that would be the best to tell them. They want to protect her and if the guy is really old there is something just not right about that.